Add a filter to TimberPostGetter::get_posts

Makes it easier to add support for plugins that like to mess with the loop (eg. BuddyPress). 

This way you can add support for popular plugins by writing small plugins/extensions like this:
https://github.com/slimndap/TimberBuddyPress

No need to add any hacky code to you theme anymore!
Merge branch 'patch-1' of github.com:slimndap/timber into slimndap-patch-1

* 'patch-1' of github.com:slimndap/timber:
  Add a filter to TimberPostGetter::get_posts
tweaked filter name and gave one to get_post as well