R1:e8744e7eeb55

Enable pygment highlighting for diffusion

Summary:
set the config for the diffusion highlighter. In D202 we
enabled it for differential already.

Test Plan:
opened a python file in diffusion and verified it is highlighted.

Reviewed By: epriestley
Reviewers: epriestley, tuomaspelkonen
CC: aran, epriestley
Differential Revision: 260