Handle that the comment marker can contain regex

In the case of e.g. C Double-tap will correctly detect * as a potential
character a comment can start with. But * has special meaning in regular
expressions and in the case of Vim will trigger an error if not used
correctly.

Protect what Double-tap find by surrounding it in \V...\m. Thanks to
@utrack for the idea.

Fixes #5.

plugin/double-tap.vim

@@ -53,7 +53,7 @@ function! s:Detect_empty_comment()
     let s:commStart[&ft] = matchstr(&comments, s:pattern)
   endif
   let line = getline('.')
-  if s:commStart[&ft] != '' && line =~ '^\s*'. s:commStart[&ft] . '\s*$'
+  if s:commStart[&ft] != '' && line =~ '^\s*\V'. s:commStart[&ft] . '\m\s*$'
     return "\<C-U>"
   else
     return "\<CR>"