Add command to Proc

In order to be able to independently execute procs this change adds an extra struct member called `Cmd`. Which is stored along with the proc under it's own path. This content of this path can be used on retrieval to execute the proc with the given command. This should decouple the Proc from the contents of Procfiles and should allow scenarios in the future like on-off tasks achieved with temporary Procs.

***
The commits in this branch will be cleaned up in time and likely stashed to a single coherent change-set
/rfr @bernerdschaefer @grobie @juliusv