Option to close preview after leaving insert mode

Added the `g:ycm_autoclose_preview_window_after_insertion` option
(similar to `g:ycm_autoclose_preview_window_after_completion`).
If set, the preview window is automatically closed when the user leaves
insert mode.
Merge pull request #189 from fwalch/autoclose_preview_window_after_insertion

Added option to close the preview window after leaving insert mode