Support for buttercup password manager

[fiendUnpleasant]

I would love to have the ability to use the buttercup password manager.

[PalmerAL]

That sounds reasonable to me, although I have a bit of a concern that it may not be popular enough for the integration to receive much usage. They have a library that should make this very feasible: https://github.com/buttercup/buttercup-core

Would be happy to take a PR for this if anyone's interested.