Ability to invite users to live threads

[markspolakovs]

It seems like the https://www.reddit.com/dev/api/#POST_api_live_{thread}_invite_contributor method and friends are missing.

I can have a crack at contributing them, but this would be my first contribution to a Ruby gem.

[markspolakovs]

In fact, I have a question - it seems like it would need a helper to process permissions, to convert, for example, [:manage, :update] into +update,+manage which the API expects. What would be the best place to put such a helper? (I think the subreddit moderator APIs use the same syntax.)