Suggestion: wait-on systemd / ps aux

Currently to wait for a process to be up you can write while loop:
```bash
while `ps aux | grep [p]ostgres`; do sleep 1s; done #syntax i should always google
```
But it would be nifty to do it like:

`wait-on process:postgres` for `ps | grep` alike
and
`wait-on systemd:postges` for ` systemctl status postgres.service | grep running`

As a proposed solution for: https://github.com/jeffbski/wait-on/issues/15

No files or tests are identified in the issue. Start by tracing the existing wait-on resource handling and CLI syntax, then assess how process and systemd checks would fit. Done means supporting the proposed process:postgres and systemd:postgres forms with reliable waiting behavior.

Written by the indexing model from the issue text.