Allow rulesets to define which file extensions to check

Let a ruleset specify default file extensions. Can still be overridden by the command line.

When a standard/ruleset knows about common file extensions there is no need to repeat them on the command line each time.

Example:

```xml
<ruleset>
 <!-- other ruleset stuff -->

 <extension>php</extension>
 <extension>inc</extension>
 <extension>test</extension>
 <extension>js</extension>
 <extension type="JS">es</extension>
</ruleset>
```