simple-markdown: add basic link support

Test Plan:
no tests yet (need to catch a ride), but:
[this link](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%5Cn%5Cn%5B%5B%E2%98%83%20passage%201%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%22let%27s%20test%20paragraohs%5Cn%5Cnthis%20is%20a%20new%20paragraph.%20i%20can%20%2Aitalic%2A%20things%20and%20%2A%2Abold%2A%2A%20things.%20%60code%60%5Cn%5Cn%5Bhere%20is%20a%20link%5D%28http%3A%2F%2Fwww.google.com%29%22%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%2C%22passage%201%22%3A%7B%22type%22%3A%22passage%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22passageTitle%22%3A%22%22%2C%22passageText%22%3A%22%22%2C%22footnotes%22%3A%22%22%2C%22showLineNumbers%22%3Atrue%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)
should work!

Reviewers: emily

Reviewed By: emily

Subscribers: kevin, tony

Differential Revision: http://phabricator.khanacademy.org/D12498