Allow beta versions for specified apps

[topbanana]

Some apps have important new features in their betas.
Some apps are no longer being developed, but the last release happened to be a beta (e.g. Winamp 5.8 beta)
Some apps are developed really, really slowly (e.g. Teracopy)
Some apps just remain in perpetual beta... forever...

If we could at least have a per app 'Enable Betas' option... It could include a disclaimer, etc.
A 'Globally enable betas' option might be included for completeness, but the per app one is enough i'd guess.
And this would allow us to deal with any extraordinary circumstance that requires it, whatever the reason is.