Feat/extras

This patch adds support for adding additional `/etc/hosts` entries, which aren't present in vagrant. It is necessary for VIP's, or for simply listing hosts that you want to include in your `/etc/hosts` faked "DNS".

This patch is based off of my earlier feature: https://github.com/smdahlen/vagrant-hostmanager/pull/109

If you'd like it rebased off of master, I can do that, but it makes sense to merge the other one first, and then this one.

Cheers,
James