Newt runner improvements

[rsdoiel]

Right now newt can run both newt router and mustache engine. To make development convenient it would be nice to have it also manage starting and stopping PostgREST when the scheme/sql get changed or data base reloaded. To be able to do that I would need to be able to do the following

• launch PostgREST with the right config file tracking the pid of the process
• newt would need to handle signals like SIGTERM and SIGHUP with notification also sent to PostgREST.
• newt would need to handle SIGKILL with a message that indicates the PostgREST process is could still be running.