Add possibility to disable identifier completion engine

Setting negative value of 'g:ycm_min_num_of_chars_for_completion'
disables identifier completion.

README.md

@@ -656,8 +656,8 @@ option is set to `2`, then when the user types a second alphanumeric character
 after a whitespace character, completion suggestions will be triggered. This
 option is NOT used for semantic completion.
 
-Setting this option to a high number like `99` effectively turns off the
-identifier completion engine and just leaves the semantic engine.
+Special value `-1` disables identifier completion engine and just leaves the
+semantic engine.
 
 Default: `2`
 

autoload/youcompleteme.vim

@@ -520,6 +520,10 @@ function! s:InvokeCompletion()
     return
   endif
 
+  if g:ycm_min_num_of_chars_for_completion < 0
+    return
+  endif
+
   if s:InsideCommentOrStringAndShouldStop() || s:OnBlankLine()
     return
   endif