Add typogr.js

This adds [typogr.js][1] (a set of functions which automatically apply various transformations to plain text in order to yield typographically-improved HTML).

Originally, I had intended to write this as a content plugin but couldn't find an easy way to get at the rendered HTML (post markdown conversion).

Note, this is dependent on [some][2] [patches][3] to `marked` and hence may not be suitable to merge but is here for reference.

  [1]: https://github.com/ekalinin/typogr.js
  [2]: https://github.com/chjj/marked/pull/83
  [3]: https://github.com/chjj/marked/pull/96