auto-config: set completeopt only if it's the default

The setting is also moved from "ftplugin" to "plugin", which allows for
easier customization, e.g. via a FileType plugin.

Ref: https://github.com/davidhalter/jedi-vim/issues/374#issuecomment-97621368

README.rst

@@ -113,7 +113,7 @@ you disable the auto-initialization in your ``.vimrc``:
     let g:jedi#auto_initialization = 0
 
 There are also some VIM options (like ``completeopt`` and key defaults) which
-are automatically initialized, but you can change all of them:
+are automatically initialized, but you can skip this:
 
 .. code-block:: vim