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
7c9c3284ed