This helps with reducing redundant config.
Note: There is no default for `alert_every` because the zero value has a
meaning and cannot be interpreted as an omission.
This makes the configuration more similar to Minitor-py and
docker-compose. If a string is passed, it will be executed in a shell.
If an array is passed, it will be executed in as a command directly.
This breaks compatiblity with previous versions of Minitor-go, but
closer to compatiblity with Minitor-py.
