Expose `--coverage` and `--no-coverage` flags from `arc diff`

Summary: Currently, we don't expose these at top level, so you can't disable coverage if your coverage is explosively broken. Expose them as passthrough arguments.

Test Plan:

  - Touched a file in `arc` which triggered unit tests.
  - Without `xdebug` installed:
    - Ran `arc diff --preview`, `arc diff --preview --no-coverage` (both fine).
    - Ran `arc diff --preview --coverage`, got exception about coverage not being available.
  - Installed `xdebug`.
    - Ran `arc diff --preview`, got coverage.
    - Ran `arc diff --preview --coverage`, got coverage.
    - Ran `arc diff --preview --no-coverage`, no coverage.

Reviewers: indiefan, btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4745