Ability to reorder columns/rows manually

Select rows/columns (e.g. with click-drag/shift-click/command-click/etc) and then drag them to a different location (indicated by a triangle that shows up near the cursor between 2 rows/columns with a highlighted line down the row/column).

Note, this feature would have to be disabled for either columns or rows if the columns or rows have been clustered. However, I'll be submitting another feature request dealing with reordering rows/cols using the trees.