Allow user id lookup via email

[gnmerritt]

Thanks for the great utility!

This PR adds support for searching for a user id by their email. Not sure what you want to do about the new integration test but I figured I'd stick in your public github email in it for now.

I'm seeing a bunch of failures on make test even w/o my changes, so I assume there must be extra environment needed to run them cleanly. Please let me know if that's not true!

[gnmerritt]

Updated when I moved from my sandbox to the real slack I was targetting - finding user ids by going through lists breaks if slack paginates the response so I switched to using the dedicated lookup endpoint and im.open with the user id