No one thought we could create widgets this way. Then this happened.

Summary:
Adds a silly shorthand syntax for passage-refs in radios, and
adds the appropriate interwidget communication so that they
work.

This is a hacky way to do this. There are potentially better
ways to do this but they involve a large amount of renderer
refactoring, and while I would like to do them I don't have
time yet. The syntax can be the same later. But I think it's
more valuable to get this out and change it later if
necessary :(.

Test Plan:
Visit [this item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20passage%202%5D%5D%5Cn%5Cn%5B%5B%E2%98%83%20group%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%22passage%202%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%22Lorem%20Ipsum%20is%20%7B%7B__simply%20dummy%20text%20of%20the%20printing%20and%20typesetting%20industry.%20Lorem%20Ipsum%20has%20been%20the%20industry%27s%20standard%20dummy%20text%20ever%20since%20the%201500s__%7D%7D%2C%20when%20an%20unknown%20printer%20took%20a%20galley%20of%20type%20and%20scrambled%20it%20to%20make%20a%20type%20specimen%20book.%20It%20has%20survived%20not%20only%20five%20centuries%2C%20but%20also%20the%20leap%20into%20electronic%20typesetting%2C%20remaining%20essentially%20unchanged.%20It%20was%20popularised%20in%20the%201960s%20with%20the%20release%20of%20Letraset%20sheets%20containing%20Lorem%20Ipsum%20passages%2C%20and%20more%20recently%20with%20desktop%20publishing%20software%20like%20Aldus%20PageMaker%20including%20versions%20of%20Lorem%20Ipsum.%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%2C%22group%201%22%3A%7B%22type%22%3A%22group%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20radio%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22radio%201%22%3A%7B%22type%22%3A%22radio%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22choices%22%3A%5B%7B%22content%22%3A%22%7B%7Bpassage-ref%202%201%7D%7D%22%2C%22correct%22%3Atrue%7D%2C%7B%22correct%22%3Afalse%7D%5D%2C%22randomize%22%3Afalse%2C%22multipleSelect%22%3Afalse%2C%22displayCount%22%3Anull%2C%22noneOfTheAbove%22%3Afalse%2C%22onePerLine%22%3Atrue%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%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)
which features a passage-ref inside a group inside the renderer.

See that the passage-ref's line numbers are correct.

Reviewers: michelle

Reviewed By: michelle

Subscribers: alpert, alex, dylan

Differential Revision: http://phabricator.khanacademy.org/D13766