Allow Radarr3 to use custom update channels#144

In June 2021, Radarr stopped providing regular updates to its master branch. It is however still very active in the develop/nightly branches. It would be a great feature to allow users to choose which branch to pull from on install/reinstall so we can continue to receive the updates we would like to receive.

This feature can also apply to Sonarr, Lidarr, Readarr, and possibly others.