[FR] Add a way to downgrade to the lastest stable release from a beta release

[mProjectsCode]

The beta manifest is very cool and I am using it for canary builds for Meta Bind. But I haven't found a way in brat to downgrade back to the latest stable release from a beta release.

I think this would be a cool feature as it would allow a quick workflow of testing the latest beta release and then swapping back to stable for actual use.

[TfTHacker]

This is interesting idea and I am thinking about its implementation. If someone deletes the beta plugin from the beta list, the actual plugin is not deleted. Then when a regular check for plugin updates is run, the difference should be caught and I think the most current release would be downloaded.

[mProjectsCode]

I think the easiest way would be to look at the main manifest file and download the release it points at