Visual mode for Swap commands

[Nekketsu]

Would it be possible to add support for the Swap commands in visual mode?

I think that sometimes it's useful to have a visual indicator of what we are going to swap.
I'd like to select visually a function, and being able to swap it with another one, while having the original one selected. It is the way that syntax-tree-surfer used to work, and I think it was quite usefull.

[aaronik]

Note this relates to #39 - seems to be the same underlying desire