[TODO] Make cutting branch prefixes configurable

[webknjaz]

Currently, backporting into stable-1.0 requires using backport-stable-1.0 label... But it'd be cleaner if we could support backport-1.0 instead.

This would require a config option stating that stable- is a target prefix and we'd just add it when figuring out the target branch.
Similarly, we could also make backport- label prefix configurable (the thing we cut off from the label).

Depends on: #5