Enable multiline select for blobs.

Holding shift will allow users to click and select multiple lines.
The behavior is similar to GitHub.

Interesting thing, was the parsing and highlighting already worked, but there was no interface. You had to manually specify the range in the URL hash (i.e. #L3-11). This will update the URL as the user holds shift and clicks a line number.

![multiline-select](https://f.cloud.github.com/assets/2117996/1633929/90372b08-5791-11e3-996d-320ac1f91458.png)

The selection behavior mimics GitHub's near as I can tell.