Allow user to specify a default sort order

We needed the ability to have the first click on a column header sort descending, rather than ascending. We found it wasn't possible with the library as it was, so we made a little patch that allows the user to supply a new data attribute, `data-default-sort`. If that is not supplied, it works just as it used to, sorting by ascending on the first click.

We didn't add any docs or re-minify this version, FYI.