feat: add ability for finders to provide an options transformer

So that specific finders can override options dynamically/temporarily.
For example, the finders in this commit have been updated to always show
dotfiles, because that is the logical behavior for items that are not on
the filesystem (leading dots only have that special meaning in the
filesystem).

Example: try searching for `netrwbook` (to find `.netrwbook`) in
`:CommandTHelp` before and after this commit.

Author: wincent

lua/wincent/commandt/init.lua

@@ -44,6 +44,10 @@ local options_spec = {
             },
             optional = true,
           },
+          options = {
+            kind = 'function',
+            optional = true,
+          },
           command = {
             kind = {
               one_of = {
@@ -222,6 +226,15 @@ local options_spec = {
   end,
 }
 
+-- Function that returns a copy of `options` with
+-- `always_show_dot_files = true` and `never_show_dot_files = false`.
+local force_dot_files = function(options)
+  return merge(options, {
+    always_show_dot_files = true,
+    never_show_dot_files = false,
+  })
+end
+
 local default_options = {
   always_show_dot_files = false,
   finders = {
@@ -243,6 +256,7 @@ local default_options = {
         end
         return paths
       end,
+      options = force_dot_files,
     },
     find = {
       command = function(directory, options)
@@ -347,6 +361,7 @@ local default_options = {
         -- context, see: https://github.com/autozimu/LanguageClient-neovim/pull/731
         vim.cmd('try | ' .. command .. ' ' .. item .. ' | catch /E434/ | endtry')
       end,
+      options = force_dot_files,
     },
     line = {
       candidates = function()
@@ -366,6 +381,7 @@ local default_options = {
         local index = tonumber(item:sub(suffix))
         vim.api.nvim_win_set_cursor(0, { index, 0 })
       end,
+      options = force_dot_files,
     },
     rg = {
       command = function(directory, options)
@@ -560,6 +576,12 @@ commandt.finder = function(name, directory)
   if config == nil then
     error('commandt.finder(): no finder registered with name ' .. tostring(name))
   end
+  if config.options then
+    -- Optionally transform options.
+    local sanitized_options, errors = sanitize_options(config.options(options))
+    report_errors(errors, 'commandt.finder()')
+    options = sanitized_options
+  end
   if directory ~= nil then
     directory = vim.trim(directory)
   end