Better support for custom Maven packaging types

[atomicknight]

It would be nice if the plugin could support custom Maven packaging types via a mapping mechanism. For example, maven-ear-plugin provides a configuration setting for mapping custom types to known types: http://maven.apache.org/plugins/maven-ear-plugin/modules.html#Custom_Artifact_Types

The currently-implemented packaging configuration setting works on a per-module basis, but builds that use custom types extensively would benefit from the ability to configure this setting in one place.

[atomicknight]

Created pull request #13.