Add an option to support optional regex input

Fix justinmk#104. This should support more flexibility in customisation. With
this option enabled, input text not matching '^\\[vVmM].' are still
treated literal.

autoload/sneak/search.vim

@@ -8,7 +8,9 @@ func! sneak#search#new()
     " search pattern modifiers (case-sensitivity, magic)
     let self.prefix = sneak#search#get_cs(a:input, g:sneak#opt.use_ic_scs).'\V'
     " the escaped user input to search for
-    let self.search = escape(a:input, '"\')
+    let input = escape(a:input, '"')
+    let self.search = g:sneak#opt.regex_input && input =~? '^\\[vm].' ?
+          \ input.'\V' : escape(input, '\')
     " example: highlight string 'ab' after line 42, column 5 
     "          matchadd('foo', 'ab\%>42l\%5c', 1)
     let self.match_pattern = ''

plugin/sneak.vim

@@ -24,6 +24,7 @@ func! sneak#init()
       \ ,'use_ic_scs'   : get(g:, 'sneak#use_ic_scs', 0)
       \ ,'map_netrw'    : get(g:, 'sneak#map_netrw', 1)
       \ ,'streak'       : get(g:, 'sneak#streak', 0) && (v:version >= 703) && has("conceal")
+      \ ,'regex_input'  : get(g:, 'sneak#regex_input', 0)
       \ }
 
   for k in ['f', 't'] "if user mapped f/t to Sneak, then disable f/t reset.