Feature Request: Forums

Going here at request of @flaviocopes from github

Hi all!

Just discovered grav and has found it extremely versatile!

But after some search in the issues i dont find a little thing would be crucial for migrating my website to grav: What about a Forum plugin?

Instead of bbcode, it would use Markdown and integrate nicefully with existent themes.
Maybe a register form, and social login integrated.

I dont know how data would be stored (not a hardcore dev boy), so if this request are impossible with current architeture, sorry. And just ignore these requests if already exists.