Option to Reverse Lookup Emails

Although I do not believe displaying emails publicly is a good decision due to the large amount of email harvesting that can be done using that, looking up emails should be possible. For example, when you are encrypting a message to a user; if the "recipient" field has an exact match for a confirmed email on Keybase, then the users username should be displayed as a recipient result.

No files, tests, or entry points are named. Start by tracing the client’s recipient field and confirmed-email lookup; done means an exact match for a confirmed Keybase email produces the corresponding username as a recipient result without publicly displaying email addresses.

Written by the indexing model from the issue text.