Provide basic capabilities to make Differential column width flexible

Summary:

• Make wrap width settable in PHP.
  • Dynamically generate max-width based on configurable maximum width.
  • Constrain non-diff elements to standard width.
  • Provide a configuration setting.

Test Plan:
Set various things to 100 / 120, as far as I could tell everything seemed to
render sensibly? This should have no effect on 80-col changes.

Reviewed By: jdperlow
Reviewers: jdperlow, tuomaspelkonen, jungejason, aran
CC: aran, jdperlow
Differential Revision: 413