If a user uploads a public key block with multiple master keys, instead of failing, ask them to choose

Related to #202, when a user does `gpg --armor --export `, they'll get _all_ the public keys which correspond to that email address. Some users have more than one key; some users have one current key, and some revoked/expired ones. This results in a public key block containing multiple keys.

Right now, Keybase throws an error when this happens (#82, #170). This seems like non-ideal behavior. Instead, parse the keys first. If any are revoked or expired, discard those. If that leaves you with one valid key, use that. Otherwise, display a key-summary to the user, and ask them which one they want to attach to their Keybase profile.

Start by reproducing the issue with `gpg --armor --export ` and read related issues #202, #82, and #170 for existing behavior and constraints. No implementation files or tests are named; done means revoked or expired keys are discarded, one remaining valid key is selected automatically, and multiple valid keys produce a summary and choice prompt.

Written by the indexing model from the issue text.