Adds toggle option for data-order

Love this plugin, but really wanted a toggle functionality, so I added it :)

This will switch between desc and asc order each time it is activated.  It can be set on a sort field like the following:
<li class="sort" data-sort="data-name" data-order="toggle">Sort by Name</li>

Note:  I only modified the original source--I didn't recompile the minified version.

Edit: I realized that the original version was swapping even if it wasn't the active sort, which is probably unexpected behavior.  I fixed that to now go back to the last used sort order if not active.