Implemented language injections for strings.

This adds support for injecting languages into single-line strings and block strings.  For instance, we can inject JavaScript into a foreign import -

![image](https://cloud.githubusercontent.com/assets/3103764/5589810/2182014c-90ed-11e4-904c-f3acee7acf4f.png)

Note that escaped multiline strings are not supported.  For instance, the following does not support language injections -

```purescript
foo = "foo\
      \bar\
      \baz"
```