Sorting improvements

This release enables multi-column sort and accessing/controlling the sort state via ReactiveVars. The old sort option for fields is deprecated, and there are two new options instead: sortOrder and sortDirection.

sortOrder should be a positive number, and the primary sort will be the field with the lowest sortOrder. sortDirection should be 1 for ascending or -1 for descending. Either of these can also be a ReactiveVar, which will be updated whenever the user changes the sorting, and will update the table whenever it's changed.