Add 'arc upload' and 'arc download'

Summary: Mechanisms for interacting with Files via Arcanist.
Test Plan:

• Ran 'arc upload x', 'arc upload x y z'
• Ran 'arc download' with --as and --show.

Reviewed By: codeblock
Reviewers: codeblock, jungejason, tuomaspelkonen, aran
CC: aran, codeblock, epriestley
Differential Revision: 742