Add an appendPath method to PhutilURI

Summary: This method would primarily be useful when you have a PhutilURI (which may or may not have a path) and you would like to define a path relative to the existing URI.

Test Plan: Wrote and executed unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9823