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