Adding 'preview' to completeopt if not forbidden

The user can opt-out of this, but we set it by default since most people don't
know that the option exists. Those that are annoyed by it can just toggle an
option in their vimrc.

autoload/youcompleteme.vim

@@ -38,10 +38,6 @@ function! youcompleteme#Enable()
     return
   endif
 
-  if g:ycm_allow_changing_updatetime
-    set ut=2000
-  endif
-
   augroup youcompleteme
     autocmd!
     autocmd CursorMovedI * call s:OnCursorMovedInsertMode()
@@ -68,6 +64,14 @@ function! youcompleteme#Enable()
   " Also, having this option set breaks the plugin.
   set completeopt-=longest
 
+  if g:ycm_add_preview_to_completeopt
+    set completeopt+=preview
+  endif
+
+  if g:ycm_allow_changing_updatetime
+    set ut=2000
+  endif
+
   " With this command, when the completion window is visible, the tab key will
   " select the next candidate in the window. In vim, this also changes the
   " typed-in text to that of the candidate completion.

plugin/youcompleteme.vim

@@ -37,12 +37,16 @@ let g:ycm_min_num_of_chars_for_completion  =
 let g:ycm_filetypes_to_ignore =
       \ get( g:, 'ycm_filetypes_to_ignore', { 'notes' : 1 } )
 
+" TODO: make this more granular
 let g:ycm_filetype_completion_enabled =
       \ get( g:, 'ycm_filetype_completion_enabled', 1 )
 
 let g:ycm_allow_changing_updatetime =
       \ get( g:, 'ycm_allow_changing_updatetime', 1 )
 
+let g:ycm_add_preview_to_completeopt =
+      \ get( g:, 'ycm_add_preview_to_completeopt', 1 )
+
 
 " This is basic vim plugin boilerplate
 let s:save_cpo = &cpo