Add support for text/literate-coffeescript in the browser

This branch allows users of the browser-ready `extras/coffee-script.js` file to embed [Literate CoffeeScript](http://coffeescript.org/#literate) in web pages. For example:

```html
<script src="extras/coffee-script.js"></script>
<script type="text/literate-coffeescript">
This is a Literate CoffeeScript example.

    console.log "Success!"
</script>
```

This results in "Success!" being displayed in the JavaScript console.

Contributors are urged by `CONTRIBUTING.md` to add their own tests. There are, however, no existing tests for the browser scripts functionality except for the test wrapper itself in `tests/test.html` For that reason, I have not added any new tests. I have of course tested that the code works on the web.

**EDIT**: The original submission used `text/coffeescript+literate`. This was changed to `text/literate-coffeescript` in a later squashed commit due to a suggestion from @jashkenas.