Feature request: advanced sharing options

One of the stumbling blocks with using hackmd.io for group projects for me is a lack of advanced sharing options. What I'd like to have is something like google docs does: a "share" button, which gives the form where I can enter a collaborator email and choose his rights (read/edit/change sharing options). Currently, if you share a document to some person, you cannot control how far it spreads.

I'd much like the feature myself and be happy to contribute 20$ to the development of this feature, especially if there was an option to "buy 4 coffees" without the need to enter cc data 4-times (nor saving it in paypal).

This request is similar to #494.