Support definition of custom commands

[fago]

Extend phapp.yml to support definition of custom per-phapp commands. Prefix them to avoid name collisions, e.g. via ":command".

[fago]

by doing #28 (comment) a phapp built-in command does not really differ from a custom command any more - it just runs the given command with bash with a possibly initialized environment, via "phapp environment"

Given that, we don't need to prefix custom commands any more.