[slim-vim] Slim-vim has a "REPL"

[Brad Beveridge]

I've added support for a REPL-like buffer.  It shows you the current
package the REPL is in, you can type forms into the buffer.  When you
hit <CR> from normal mode, the outermost form is evaluated and the
result placed into the REPL.  It appears to work pretty well to me.
All Swank output that would normally go to *standard-output* now shows
up in --slim-vim-repl--.

Progress has been a bit slow lately, mostly because of time, but
partly because right now Slim-vim does most things I need.  And I
don't have a Lisp project to use Slim-vim with yet :)
So, keep at me with the feature requests!  (and remind me if I appear
to have forgotten yours)

Cheers
Brad