Highlight multiple lines

This allows highlighting multiple lines, as described in travis-ci/travis-ci#1829

We can still select a single line, like we were previously able to.
![screen shot 2014-01-07 at 12 12 23](https://f.cloud.github.com/assets/9347/1858454/b7bb1980-778c-11e3-810a-03e6d02ca1d3.png)
![screen shot 2014-01-07 at 12 12 23 2](https://f.cloud.github.com/assets/9347/1858455/b7bd3f1c-778c-11e3-8da1-f2ec378aa77e.png)

And, once a starting has been selected, we can select an ending one with the shift key.
![screen shot 2014-01-07 at 12 12 29](https://f.cloud.github.com/assets/9347/1858465/dd634838-778c-11e3-8c1c-723ed578efe0.png)
![screen shot 2014-01-07 at 12 12 29 2](https://f.cloud.github.com/assets/9347/1858464/dd4dab5e-778c-11e3-8f89-3446abb8b8b8.png)