Support regex input mode

Prefix input text with '\v', '\m' or '\M' to turn on "regex mode".

This doesn't break search of "literal text" whose length is less than 3.
Because only text matching '^\\[vVmM].' are not literal any more, which
is identical to what "literal text" means in Vim search pattern starting
with '\V'.

Systemic adjustments are needed for utilizing "regex mode", which is
better to be served by the author.

Close justinmk#104.

autoload/sneak/search.vim

@@ -8,7 +8,8 @@ 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 self.search = a:input =~? '^\\[vm].' ?
+          \ escape(a:input, '"').'\V' : escape(a:input, '"\')
     " example: highlight string 'ab' after line 42, column 5 
     "          matchadd('foo', 'ab\%>42l\%5c', 1)
     let self.match_pattern = ''