Lazily require readline

[deivid-rodriguez]

The current implementation of readline has issues under Windows, and those issues sometimes 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.

Just to be clear, this is not a problem with thor at all, but since requiring unnecessary things lazily is usually a good thing, I figured we could do this.

Also, an alternative version of this patch would be to autoload Thor::LineEditor, like

diff --git a/lib/thor/base.rb b/lib/thor/base.rb
index 26132de..3bbbbd2 100644
--- a/lib/thor/base.rb
+++ b/lib/thor/base.rb
@@ -4,7 +4,6 @@ require_relative "error"
 require_relative "invocation"
 require_relative "parser"
 require_relative "shell"
-require_relative "line_editor"
 require_relative "util"
 
 class Thor
diff --git a/lib/thor/shell/basic.rb b/lib/thor/shell/basic.rb
index 49e4cb2..9a48e8b 100644
--- a/lib/thor/shell/basic.rb
+++ b/lib/thor/shell/basic.rb
@@ -1,4 +1,6 @@
 class Thor
+  autoload :LineEditor, "thor/line_editor"
+
   module Shell
     class Basic
       DEFAULT_TERMINAL_WIDTH = 80