Add a `phutil_var_export` function

Summary: Add a `phutil_var_export` function which behaves similarly to `var_export(..., true)`, except that the output is nicer on the eyes.

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/D9923