Allow selection of subkey

By default, keybase seems to invoke gpg something like `gpg -u `. gpg then selects which subkey to sign with by looking for the most recently created subkey with usage S. Obviously this is not always the right key to use; selecting a different subkey is done by invoking gpg like `gpg -u !` (note the trailing exclamation mark to force the use of the specific subkey that is named). This applies to all of the signing operations: `keybase prove`, `keybase sign`, etc.

I think there is also a similar issue with `keybase encrypt`; the recipient may have multiple encryption subkeys, in which case gpg will just select the most recently created encryption subkey, but this is not necessarily the one you want to use. If you know that you want to use a different encryption subkey, there should be a way to specify this.

Start by tracing the signing operations named in the issue—keybase prove and keybase sign—and the keybase encrypt path to see how each invokes gpg. Confirm how gpg receives the master or subkey identifier, then define how users should select a specific signing or encryption subkey. Done means all listed operations consistently honor an explicitly selected subkey.

Written by the indexing model from the issue text.