Variant of edit that runs just the sandbox.

Suitable for running inside split panes atop tmux so you can run vim or some
other editor with mu. For example, here's mu running alongside vim atop tmux:

<img alt='tmux+vim example' src='../html/tmux-vim-sandbox.png'>

To set this up:

  a) copy the lines in tmux.conf into $HOME/.tmux.conf

  b) copy the file `mu_run` somewhere in your $PATH

Now when you start tmux, split it into two vertical panes, run `mu sandbox` on
the right pane and your editor on the left. You should be able to hit F4 in
either side to run the sandbox.

Known issues: you have to explicitly save inside your editor before hitting
F4, unlike with 'mu edit'.