Lazily require readline

The current implementation of readline has issues under Windows, and
those issues prevent thor from being used. For example, in our bundler
Windows CI, we had to patch `rb-readline` to workaround some issues
because `rb-readline` (the default `readline` provider on Windows)
couldn't be required. Moreover, we still get some issues when requiring
`rb-readline` like

```
HOME environment variable (or HOMEDRIVE and HOMEPATH) must be set and point to a directory
```

Since in some situations the part of thor using `readline`
(`Thor::LineEditor`) is not required, for example, in most of CI
situations. I thought it could be a good idea to lazily require
`readline`.

Author: deivid-rodriguez

lib/thor/line_editor/readline.rb

@@ -1,12 +1,12 @@
-begin
-  require "readline"
-rescue LoadError
-end
-
 class Thor
   module LineEditor
     class Readline < Basic
       def self.available?
+        begin
+          require "readline"
+        rescue LoadError
+        end
+
         Object.const_defined?(:Readline)
       end