Add some create mail handlers for paste and files

Summary: Fixes T1144. Though actually I think T1144 wanted some handy way to email from the command-line / arc, this is cooler. :D

Test Plan: set conf properly and then ./bin/mail receive-test --as btrahan --to pasties@phabricator.dev | README --> it worked...! couldn't test files as easily but verified exception thrown when I tried to test.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1144

Differential Revision: https://secure.phabricator.com/D6622