un-backslashify-selection: Handy X hack.

Now I'm using Emacs for reading email again, there's a small problem.
Long URLs are broken across lines with a backslash/newline pair.  This
makes them a bit tricky to paste into Iceweasel or whatever.

un-backslashify-selection reads the selection, kills the backslashes,
and rewrites the selection.  This command is bound to M5-F7, so pasting
is fairly painless.  (An extension is the right answer, but that's way
more work.)