Add the ability to append to $PATH, for when we shell out to system binaries.

Summary:
In some cases, we shell out to things (like Pygments for syntax highlighting).
However, on cloud servers or shared web servers, those binaries aren't always
installed system-wide.

This patch allows for appending to the environment variable $PATH, to look for
other, non-default places for these binaries.

Test Plan:

• Copied the patch over to a test OpenShift instance and applied it.
• Added the path to my local copy of Pygments (pygmentize wasn't available on the system) into the Phabricator config.
• Refreshed a Paste page, and saw colors.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3091