Add support for config file and minimum and maximum values in docs

Allows you to supply a config file via `prmd -s prmd-config.json` and override any of the other settings you would supply. This also supports YAML config files.

Couple of extra bonus things:
* Remove the url style parameter from the command line and only supply it via the config file.
* Add support for `minimum` and `maximum` parameters to be displayed in the docs
* Little bug fix 
Hmm. It is an interesting question. I think there is probably value in having it in both places, so maybe we can leave it for now (and we can always review/remove later if it turns out not to be that useful).