Support for validating other XML related files (i.e. assembly descriptors).

[madelaney]

I'd like to use this plugin but to use this we'd like to be able to also check our assembly descriptors. I'd be happy to do the work if you're open to this behavior.

I was thinking of something akin the following for the configuration:

<plugin>
	<groupId>org.codehaus.mojo</groupId>
	<artifactId>tidy-maven-plugin</artifactId>
	<version>1.1.0</version>
	<configuration>
		<includes>
			<include>pom.xml</include>
			<include>src/assembly/*.xml</include>
		</includes>
	</configuration
</plugin>