Add option to discard temporary authentication

[maxgoedjen]

@taxilian I'm gonna change this one to a feature request – it currently does not behave this way, but I may add the ability to do so in the future (or otherwise manually expire the auth somehow).

My thinking is this:

• If the user unlocks the screen, they could've just re-unlocked the key.
• Building on that, locking the screen is currently just kind of a proxy for intent here – I don't know that it's intuitive enough to rely on (vs just having a "relock" button or something).
• I'm not 100% I will be able to respond to screen locks with the reliability I'd need to be comfortable with this behavior.

[maxgoedjen]

FWIW it's a hack, but if you explicitly need this behavior right now, killing SecretAgent will flush out any in-memory authorizations.

[taxilian]

Fair enough to my mind =]