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