Add auto yank featrue

The following new options are added.

-   g:gista#gistid_yank_format (Default: '{gistid}')
-   g:gista#gistid_yank_format_with_file (Default: '{gistid}/{filename}')
-   g:gista#gistid_yank_format_in_post (Default: 'GistID: {gistid}')
-   g:gista#gistid_yank_format_in_save (Default: 'GistID: {gistid}')
-   g:gista#auto_yank_gistid_after_post (Default: 1)
-   g:gista#auto_yank_gistid_after_save (Default: 1)

'GistID: XXXXXX' will be yanked after the user post/save the gist.
Bump version