R1:7c9c3284ed0d

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…