Added JSON and XML file validation

A list of validators can now be added to the `configuration` section of the maven plugin.

Usage:
```xml
<validators>
   <validator>
      <type>JSON</type>
      <includeRegex>.*\.json</includeRegex>
   </validator>
   <validator>
      <type>XML</type>
      <includeRegex>.*\.xml</includeRegex>
   </validator>
</validators>
```