Add Team Interactively

[jonthegeek]

As a user, I'd like to load a team interactively with an httr browser launch, so that I don't have to work too hard to get this set up.

This should always load the team into the .slack environment, but also (optionally) save a cached version for future loading.

Make sure that no downstream functions require the file (change them if they do).

[jonthegeek]

Actually I think this should be an adaptation of add_team.