Added an action to TimberPost::init()

This action replaces the filter in TimberPostGetter::get_posts(), which should be removed again.
This way Timber::get_post() and new TimberPost() are analogues again.

It also made it much simpler for me to achieve the same results with the BuddyPress plugin. 

To see it in action:

1. Merge this pull request.
1. Install the BuddyPress plugin.
1. Activate the Timber starter theme.
1. Enable pretty permalinks.
1. View one of the BuddyPress pages (eg. example.com/groups).