How to master perseus tables in 60 seconds

Summary:
Adds title support to perseus tables, which looks like:

    || **Here is a table title!** ||
    header 1 | header 2 | header 3
    - | - | -
    data 1 | data 2 | data 3
    data 4 | data 5 | data 6
    data 7 | data 8 | data 9

I'm not totally sure I like this over making a table-output widget,
but this makes adding titles to existing tables easy, which might
be good.

Test Plan:
View [an item with a titled table](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%7C%7C%20%2A%2AHere%20is%20a%20table%20title%21%2A%2A%20%7C%7C%5Cnheader%201%20%7C%20header%202%20%7C%20header%203%5Cn-%20%7C%20-%20%7C%20-%5Cndata%201%20%7C%20data%202%20%7C%20data%203%5Cndata%204%20%7C%20data%205%20%7C%20data%206%5Cndata%207%20%7C%20data%208%20%7C%20data%209%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%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)

Delete the title and see that you still get a table.

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D14091