simple-markdown: Add character escaping

Summary:
Allows you to escape markdown special characters with backslashes.

Also adds escaping and strikethrough to passages (not tested yet).

Test Plan:
`make test`
and [view this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20simple-markdown-tester%202%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fda8df81c78b22f5c69d477d8eabfb583968eaf84.png%22%3A%7B%22width%22%3A400%2C%22height%22%3A70%7D%2C%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb59fc02ca1aae800977b8793ed22f647a1aa75ee.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A150%7D%7D%2C%22widgets%22%3A%7B%22simple-markdown-tester%202%22%3A%7B%22type%22%3A%22simple-markdown-tester%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22correct%22%3A%22%22%2C%22value%22%3A%22%5C%5C%60hi%5C%5C%60%5Cn%22%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)
and see that hi is displayed between backticks

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D12496