Allow PhabricatorPropertyListView to have associated content

Summary:
Some objects (like PhamePost and ManiphestTask) have a block of text/remarkup which serves as a description or core piece of content for the object.

Accommodate this in PhabricatorPropertyListView.

(This is primarily to let me do a reasonable first pass on this in Phame.)

Test Plan: Made example, will attach screenshot.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1373

Differential Revision: https://secure.phabricator.com/D3699