Clean up Phabricator interface to syntax highlighting

Summary: Reduce the amount of code duplication here and allow for an override
configuration on the filename.map stuff.
Test Plan: Checked paste, diffusion and differential syntax highlighting and
everything appeared reasonable.
Reviewed By: codeblock
Reviewers: tuomaspelkonen, codeblock, jungejason, aran
CC: aran, codeblock, epriestley
Differential Revision: 601