Allow p.crypt accept to use Github keys

[robdmc]

Add a new switch to the p.crypt tool.

--github [username]

If username not specified, encrypts with public key in ~/.ssh If username specified, ecrypts with that github users public key

When decrypting, uses private key in ~/.ssh

Basically, I want to wrap this cool gist by @rgbkrk

https://gist.github.com/rgbkrk/7827691

[rgbkrk]

Whoa this pandashells tool is cool. OMG.

@smashwilson and I made plans to use the newer cryptography library from Python for this (as a Python helper library). Seems like you'd want a handy library for that. I'm going to have to dig to see what p.crypt is up to. :smile: