Add command to reorder screens

[Phil Crosby]

Often times I'd like to re-order the tabs. I thought elscreen-swap might do the trick, but it seems to put elscreen into a bad state which is a bit hard to describe. After running elscreen-swap via M-x, the windows become defocused and tab switching no longer switches tabs.

@knu, I'm not sure if you're adding features, but I thought this would be as good a place as any to record the idea.