Move script out of ARGV

It's annoying that the whole shell script shows up as an argument to `sh
-c` in tools like `ps`.  Instead, dump the script into a temp file.

    $ ps -faux | awk -F\\ '/mux/&&/_ sh/ {$1="";print $0}'

    # before:
    _ sh -c #!/bin/zsh tmux start-server\; has-session [...1475 more chars...]

    # after:
    _ sh /tmp/tmuxinator20140731-11503-1bnc7e1