Allow users to provide alt text for image embeds

Summary:
Ref T4843.

  - Add an `alt` attribute so users can provide alternate text for `{Fnnn}`.
  - Add an `alt` attribute to image macros.

Test Plan: Embedded an image with `alt` and a macro, inspected HTML source to verify the `alt` attribute was present.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4843

Differential Revision: https://secure.phabricator.com/D8925