Use parallel syntax highlighting API in differential

Summary:
Use the new API from D322 to highlight text in parallel in Differential.

Test Plan:
Verified that pygemntize calls started within 20ms of one another in DarkConsole
(also: added a feature to let me do this) instead of running serially.

Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran
Differential Revision: 323