added commit message field class

Summary:
commit message fields were previously stored as name/value
pairs in an associative array. this resulted in ad hoc code to modify
the structure/rendering of these fields in commit messages. this diff
introduces a new DifferentialCommitMessageField class.

Test Plan:
ran "arc amend" to ensure the commit message still looked good

Reviewed By: epriestley
Reviewers: epriestley
CC: aran, dpepper, epriestley
Differential Revision: 554