Fix #313 (substring_match for Lisp): use simple=true

We can't feed arbitrary prefixes to substring_match match in
non-simple mode. We run into problems when the buffer has alternative
word rules (Like Lisp buffers with vlime).

The problem arises when prefix ends with % like for instance `foo%`.

Author: theHamsta

lua/completion/matching.lua

@@ -30,7 +30,7 @@ end
 
 local function substring_match(prefix, word)
   prefix, word = setup_case(prefix, word)
-  if string.find(word, prefix) then
+  if string.find(word, prefix, 1, true) then
     return true
   else
     return false