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