Add a method to PhutilConsoleTable to hide the header.

Summary: Add a setShowHeader method to PhutilConsoleTable so that the header row can be hidden.

Test Plan:
Constructed a headerless table.

+==========+==========+=====================+
|    12345 | alicoln  | abraham@lincoln.com |
| 99999999 | jbloggs  | joe@bloggs.com      |
+==========+==========+=====================+

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9596