ycm_cache_omnifunc should use the user's value

plugin/youcompleteme.vim

@@ -133,7 +133,8 @@ let g:ycm_semantic_triggers =
       \   'erlang' : [':'],
       \ } )
 
-let g:ycm_cache_omnifunc = 1
+let g:ycm_cache_omnifunc =
+      \ get( g:, 'ycm_cache_omnifunc', 1 )
 
 " On-demand loading. Let's use the autoload folder and not slow down vim's
 " startup procedure.