R1:4d12d014fd9d

Add PhutilRemarkupHexColorCodeRule, a new remarkup rule to format color codes

Summary:
Implements a remarkup rule that formats the background color to match the value of inline hex color codes.

The format for this rule looks like `{#RRGGBB}` and it will be formatted the same as monospace text, so `#RRGGBB` with the background set to the specified color.

Test Plan:
* run `arc unit` and see the tests pass
* create some markup with {#ff0000} color codes for all of your favorite colors {#ee33ee}

Reviewers…