Make HTML email a little easier to debug

Summary:
Ref T992.

  - Format text/HTML bodies explicitly in `bin/mail show-outbound`.
  - Provide `bin/mail show-outbound --dump-html` so you can do something like `bin/mail show-outbound --dump-html > dump.html; open dump.html` to get a browser preview somewhat easily.

Test Plan: Ran `bin/mail show-outbound` with and without `--dump-html` flag.

Reviewers: talshiri, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T992

Differential Revision: https://secure.phabricator.com/D10272