Add support for customizing filtering delimiters like the resources plugin

After recently upgrading to Maven3 beta1 from alpha7 I noticed that our filters were not taking effect any more. We were using '@' for creating filer tokens, which used to work alpha7 but no more with beta1.

In any case, looking at the resources plugin, it has a delimiters property that can be used to customize the delimiters when filtering. As suggested by

http://stackoverflow.com/questions/2315009/override-mavens-default-resource-filter-replacement-pattern

I set an appropriate property on the resources plugin, but no such property exists for the WAR plugin at the moment which means I am stuck. I need to keep the delimiter token as '@' because we need to maintain Ant compatibility for the time being.

It would be nice if this property also existed on the WAR plugin so that filtering delimiters could be customized.

The code in the resources plugin that does the job:
http://maven.apache.org/plugins/maven-resources-plugin/xref/org/apache/maven/plugin/resources/ResourcesMojo.html#233