Added Eval methods for tmux to the base plugin.

The user can map these with:
xmap <leader>e <Plug>SlimeRegionEval
nmap <leader>e <Plug>SlimeMotionEval
nmap <leader>ee <Plug>SlimeLineEval

The only difference from simply sending, is that it takes advantage of
the pipe-pane command. Starting this, then sending the input, then
closing it allows us to get the results from the pane. These results get
saved into the "e"val register at the moment.

However, tmux *does not* differentiate between input and output lines,
so the user gets their input and their output. The next step is to allow
for reading over the output, and collecting only the bits necessary.
This will have to be done on a filetype/program basis, since it will
depend upon knowing how to "find" the output.