feat(tag): let :CommandTTag open stuff in splits, tabs etc

As noted in the related issue, we can't use `:tag` if we want to be able
to open stuff in splits.

The naïve approach doesn't work:

```diff
diff --git a/lua/wincent/commandt/init.lua b/lua/wincent/commandt/init.lua
index 339edd4..d9f36c9 100644
--- a/lua/wincent/commandt/init.lua
+++ b/lua/wincent/commandt/init.lua
@@ -660,7 +660,16 @@ local default_options = {
           end
         end

-        vim.cmd('silent! tag ' .. tag_name .. ' | normal! zz')
+        local tag_command = 'tag ' .. tag_name .. ' | normal! zz'
+        if ex_command == 'edit' then
+          vim.cmd('silent! ' .. tag_command)
+        elseif ex_command == 'split' then
+          vim.cmd('silent! split new | ' .. tag_command)
+        elseif ex_command == 'tabedit' then
+          vim.cmd('silent! tabedit | ' .. tag_command)
+        elseif ex_command == 'vsplit' then
+          vim.cmd('silent! vsplit new | ' .. tag_command)
+        end
       end,
       options = force_dot_files,
     },
```

Because:

> It will work most of the time, except when it doesn't.
>
> Here's how to break it:
>
> 1.  Run `:CommandTTag`. Because `:CommandTTag` uses `taglist('.')`
>     under the covers, it will search a set of files relevant to the
>     current buffer. In a project like Command-T, `:echo tagfiles()`
>     prints `['tags']`, which means that `taglist('.')` will find the
>     project-local tags list and use it. Splitting etc with the above patch
>     works.
> 2.  Open a help buffer (eg. `:help :help`, for example).
>     Now `:echo tagfiles()` prints a huge list corresponding to
>     all the help tags (eg. `['$HOME/.config/nvim/doc/tags',
>     '$HOME/.config/nvim/pack/bundle/opt/vim-abolish/doc /tags',
>     '$HOME/.config/nvim/pack/bundle/opt/undotree/doc/tags', ...]` etc). If
>     you run `:CommandTTag` and try to open in a split using one of those,
>     then the above patch will first run `:split new`, _causing the output
>     of `tagfiles()` to print something else, because you're no longer in a
>     help buffer_, which means the subsequent `:tag` command will fail.
>
> So, if you have `include_filenames` set to `true`, it will work
> because we're doing it differently (jumping to the file first, then
> searching for the right position in the file). I think the only way
> we'll get this to work with `include_filenames` set to `false` is
> going to be to not use the `:tag` command at all, which in turn means
> that commands like `:tnext` won't work.

So, unless I come up with a really clever idea, this commit means we've
got opening-in-a-split, opening-in-a-tab etc working, but at the cost of
breaking `:tnext`. Let's ship it and see if there are any complaints.

Note that in order to get this to work in the test case mentioned above,
I had to make the trailing `/` optional in the `match()` call. This is
because lines in the help tags look like this:

```
split()	builtin.txt	/*split()*
```

(Note the absence of the trailing slash.) Compare that with something
like `scanner_free` in Command-T:

```
scanner_free	lua/wincent/commandt/lib/scanner.c	/^void scanner_free(scanner_t *scanner) {$/;"	f	typeref:typename:void
scanner_free	lua/wincent/commandt/lib/scanner.h	/^#define scanner_free commandt_scanner_free$/;"	d
```

(That's generated with Universal Ctags.) Then there's what you get with
Exuberant Ctags:

```
scanner_free	lua/wincent/commandt/lib/scanner.c	/^void scanner_free(scanner_t *scanner) {$/;"	f
scanner_free	lua/wincent/commandt/lib/scanner.h	/^#define scanner_free /;"	d
``

In all cases Neovim is going to slice the relevant pattern out of the
tags file and pass it to us in the `cmd` value, so what we actually get
will be a pattern like one of the following:

- /*split()*
- /^void scanner_free(scanner_t *scanner) {$/
- /^#define scanner_free commandt_scanner_free$/

So, to match all of those, we need to look for a `/` at the start of the
line and capture everything up to the optional `/` at the end of the
line.

Closes: #439

Author: wincent

lua/wincent/commandt/init.lua

@@ -644,23 +644,15 @@ local default_options = {
       mode = 'virtual',
       open = function(item, ex_command, _directory, options, opener, context)
         local tag = context[item]
-        local tag_name = tag.name
-        if options.scanners.tag.include_filenames then
-          if tag.filename and tag.cmd then
-            opener(tag.filename, ex_command)
-
-            -- Strip leading and trailing slashes, and use \M ('nomagic'):
-            -- ie. "/^int main()$/" → "\M^int main()$"
-            local pattern = '\\M' .. tag.cmd:match('^/(.-)/$')
-            local line, column = unpack(vim.fn.searchpos(pattern, 'w'))
-            if line ~= 0 and column ~= 0 then
-              vim.cmd('normal! zz')
-            end
-            return
-          end
+        opener(tag.filename, ex_command)
+
+        -- Strip leading and trailing slashes, and use \M ('nomagic'):
+        -- ie. "/^int main()$/" → "\M^int main()$"
+        local pattern = '\\M' .. tag.cmd:match('^/(.-)/?$')
+        local line, column = unpack(vim.fn.searchpos(pattern, 'w'))
+        if line ~= 0 and column ~= 0 then
+          vim.cmd('normal! zz')
         end
-
-        vim.cmd('silent! tag ' .. tag_name .. ' | normal! zz')
       end,
       options = force_dot_files,
     },