Better support team assignments

[mattpolzin]

Right now, aside from being able to assign teams to one's own PR via the assign command, Harmony does not have very good support for team assignments. When you see how many PRs are assigned to you, it does not include assignments for teams which you belong to. Similarly, when using the contribute command, PRs for which your teams are assigned are not considered as your assignments.

It's probably worth exposing this as a setting, whether the default is to include team assignments or the other way around. It could be exposed as part of the config file, and/or it could be exposed as a flag like --exclude-teams would not consider team assignments (current behavior) but otherwise the new default would consider those assignments.

[mattpolzin]

If getting PRs assigned to a team for the current user is an additional API call then it might be better to continue not to by default but allow the extra info given a flag like --team-requests.