Search by e-mail [API, Feature Request]

Using keybase.io as a great way to check keys before sending emails is an incredible use of the service, but seeing as it's impossible to search by email it's impossible.

Something like the following would work nicely imho:
`https://keybase.io/_/api/1.0/user/lookup.json?email=joseph.szymborski@gmail.com&fields=basics`

I know e-mail isn't a proof and not verified like twitter, HN, etc, but searching by email would be know different than searching by keybase.io username in terms of security.

Start by reviewing the Keybase user lookup API entry point shown in the issue: /_/api/1.0/user/lookup.json with the email and basics parameters. Determine the project’s existing lookup and privacy behavior, then verify that the requested email search works consistently with those rules and has appropriate tests before considering it done.

Written by the indexing model from the issue text.