Handle Agreement tag

There's now an agreement tag in Appstream. This is distinct from a code license, where it's an agreement a user must agree to before using the app. While I don't anticipate many apps utilizing this in AppCenter, other repo apps may, and it is probably worth implementing for the spec.

My initial idea was a dialog when installing.

See: https://github.com/ximion/appstream/issues/188#issuecomment-385075318