Allow syntax highlighting to occur in parallel with futures

Summary:
Pygments is super slow and we do serial calls to it in Differential and
Diffusion. We can greatly improve the performance of some workflows in these
tools by parallelizing these calls.

Test Plan:
Created a script that made parallel calls to Pygments using the future-based
API.

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran
Differential Revision: 322