Fixed italic regex to work with a single underscore
Fixed italic regex to work with a single underscore

Summary: _test_ and *test* are both ways of italicizing

Test Plan: Type "* okay_then_", observe then being italicized.  type "_wha*" and "*wha_" and observe no italicizing.

Reviewers: emily

Reviewed By: emily

Subscribers: jared

Differential Revision: https://phabricator.khanacademy.org/D30302

ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus