Use process manager to start Procfile commands

Implementing chaining shell commands in the Procfile (issue #114) and propagating the TERM signal to child processes (issue #102) required more intelligence than the simple start script could easily provide. This PR uses `runsv` from the runit package as a process manager as that provides a finish script that can be used to send the TERM signal to all child processes.

Command chaining will be detected when the service definition is written on build and the commands will be called in a subshell.