Add back leading empty lines deleted by Pygments

Summary:
Pygments removes leading empty lines. It's is causing incorrect
rendering of diff, add a command line switch to disable this behavior.

Test Plan:
verified that it works for changesets with or without leading
empty lines.

Reviewed By: tuomaspelkonen
Reviewers: epriestley, tuomaspelkonen
Commenters: epriestley
CC: aran, epriestley, jungejason, tuomaspelkonen
Differential Revision: 317