When rendering Remarkup codeblocks, allow calling engines to provide a style map

Summary:
Ref T9790.

• Make the HTML version of code blocks look more like the web version.
• If the caller passes a style map, reformat the body to use style="..." instead of class="..." for HTML mail mode.

Test Plan:
Faked a style map and got some inline styles (red stuff) -- see also the "style" in the inspector:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9790

Differential Revision: https://secure.phabricator.com/D15847