Add context support to PHP highlighter

Summary:

• Detect x::y, and mark y as an attribute with context x.
• Export found contexts as a data attribute.

Test Plan: Load Diffusion. Examine DOM and click on things.

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, aran, Korvin

Maniphest Tasks: T1602

Differential Revision: https://secure.phabricator.com/D3214