Send more keys with VimuxSendKeys

This patch addresses issue #44, where @gberenfield says:

> I can't seem to paste in around 30-40 lines of code into my vimux repl.
> Anything less works fine.

I write the selected text to a temporary file, read the file with tmux,
and paste the contents into the appropriate window.

Caveats:

- When sending the text, vim prompts you with a `more` screen in which you
  must press Enter repeatedly to scroll to the end. The text is actually sent
  after you're done mashing the Enter button. I haven't found a way around
  this -- I'm new to Vimscript.