Revamp pack.rb input handling again.  I'm starting to learn ruby!, a little.

Summary:
stdin mode stays the same, which is what webapp uses.  But now if you
specify arguments on the commandline, you can specify them in the form
infile::outfile, and it will read from infile and write to outfile.
We now use that new functionality in 'make pack'.  Soon, webapp will
use it too, and the hacky stdin mode will go away.

Test Plan: make pack

Reviewers: eater

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4584