Check remote rather than just origin

[stevenjack]

Problem

Currently the remote checked against is origin/{branch_name}, lot's of people have more than one remote.

Solution

Check all tracked remotes for a branch in the current repo and show the status for each of them.

[JakeChampion]

If this were to be added, it would be nice to override with a single remote. I have some repositories with a lot of remotes (Heroku applications) where I really only care about one being up-to-date.