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 
To be honest I'm not quite sure this is actually clear. The more I look at the rendered examples the more it doesn't make sense to me.

<table>
  <tr>
    <th>Name</th>
    <th>Type</th>
    <th>Description</th>
    <th>Example</th>
  </tr>

  <tr>
    <td><strong>meta:topics:s</strong></td>
    <td><em>number</em></td>
    <td>Certainty score<br/><b>minimum:</b> &gt;= <code>0</code><br/><b>maximum:</b> &lt;= <code>1</code></td>
    <td><code>0.5</code></td>
  </tr>
</table>