Drive Differential commit message parsing through extensible fields

Summary:
I think this is the last major step -- use the fields to parse commit messages,
not a hard-coded list of stuff. This adds two primary methods to fields, one to
get all the labels they'll parse (so we can do "CC" and "CCs" and treat them as
the same field) and one to parse the string into a canonical representation
(e.g., lookup reviewers and such).

You'll need to impelement the one block of task-specific stuff I removed in
Facebook's task field:

list($pre_comment) = split(' -- ', $data);
$data = array_filter(preg_split('/[^\d]+/', $pre_comment));
foreach ($data as $k => $v) {
  $data[$k] = (int)$v;
}
$data = array_unique($data);
break;

Otherwise I think this is clean.

Test Plan:

• Called the conduit method with various commit messages, parsed fields/errors

seemed correct.

• "arc diff"'d this diff onto localhost, then updated it.
• "arc amend"'d this diff.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason, epriestley

Differential Revision: 829