Add support for global and process pre/post commands to yaml

[stuartcarnie]

What

Global pre / post commands

Global pre / post commands must run when gopm starts and stops. If the pre command returns a non-zero exit code, it should not start any processes.

Process pre / post commands

Each process can also provide a pre / post command. Prior to starting the process, gopm will run the pre command and prevent the process from starting for non-zero exit codes. On shutdown, the post command should run. Exit code can be logged but has no consequence.