Allow the `todo` workflow to add project tags.

Summary: Fixes T4418. Allows Maniphests created through the `arc todo` workflow to have projects assigned.

Test Plan:
```
> ./bin/arc --trace --conduit-uri='http://phabricator.joshuaspence.com' todo "Test project" --project foo --project bar
libphutil loaded from '/home/joshua/workspace/github.com/phacility/libphutil/src'.
arcanist loaded from '/home/joshua/workspace/github.com/phacility/arcanist/src'.
Config: Reading user configuration file "/home/joshua/.arcrc"...
Config: Did not find system configuration at "/etc/arcconfig".
Working Copy: Reading .arcconfig from "/home/joshua/workspace/github.com/phacility/arcanist/.arcconfig".
Working Copy: Path "/home/joshua/workspace/github.com/phacility/arcanist" is part of `git` working copy "/home/joshua/workspace/github.com/phacility/arcanist".
Working Copy: Project root is at "/home/joshua/workspace/github.com/phacility/arcanist".
Config: Did not find local configuration at "/home/joshua/workspace/github.com/phacility/arcanist/.git/arc/config".
Loading phutil library from '/home/joshua/workspace/github.com/phacility/arcanist/src'...
>>> [0] <conduit> conduit.connect() <bytes = 618>
>>> [1] <http> http://phabricator.joshuaspence.com/api/conduit.connect
<<< [1] <http> 1,050,487 us
<<< [0] <conduit> 1,051,585 us
>>> [2] <conduit> project.query() <bytes = 199>
>>> [3] <http> http://phabricator.joshuaspence.com/api/project.query
<<< [3] <http> 294,584 us
<<< [2] <conduit> 294,986 us
>>> [4] <conduit> maniphest.createtask() <bytes = 313>
>>> [5] <http> http://phabricator.joshuaspence.com/api/maniphest.createtask
<<< [5] <http> 637,693 us
<<< [4] <conduit> 638,098 us
Created task T6: 'Test project' at http://phabricator.joshuaspence.com/T6
```

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4418

Differential Revision: https://secure.phabricator.com/D9457