Added option to close the preview window after leaving insert mode

With `g:ycm_autoclose_preview_window_after_completion`, the preview window is auto-closed when I start typing. But, for example, when auto-completing a method I want to call, it'd be useful for the preview window to stay open so I can still see the called method's signature while I type.

To do that, I added the `g:ycm_autoclose_preview_window_after_insertion` option, which auto-closes the preview window after leaving insert mode.