Allow workflows to elect for partial parsing rather than full parsing

Summary:
This permits workflows to:

• Delegate to sub-workflows ("help" isn't quite correct but is probably good enough).
• Manually parse/manage raw argument vectors.

Test Plan:

$ ./scripts/example/subworkflow.php do echo -a -b -c
-a -b -c

Reviewers: davidreuss

Reviewed By: davidreuss

CC: aran

Differential Revision: https://secure.phabricator.com/D3552