Summary: Quora requested this (moving to S3) but it's also clearly a good idea.
Test Plan:
Ran with various valid/invalid options to test options. Error/sanity checking seemed OK.
Migrated individual local files.
Migrated all my local files back and forth between engines several times.
Uploaded some new files.
Reviewers: btrahan, vrana
Reviewed By: vrana
CC: aran
Maniphest Tasks: T1950
Differential Revision: https://secure.phabricator.com/D3808
