Support includes/excludes

Support advanced scanner configuration (e.g. includes/excludes), just like the compiler plugin does. This allows to restrict the sources the plugin scans, rationale see compiler plugin.

This also would help working around QDOX not supporting JDK 1.5 (see [1] and [2]).

If this is wanted and somebody tells me the best way, I'll gladly implement it. I looked at the code and noticed a "brute force" approach would mean adding the scanner configuration to the plugin MOJO and propagating it through MojoScanner.populatePluginDescriptor and MojoDescriptorExtractor.execute (changing the intefaces). Does not sound like the best option... The next thing would be to add the configuration to PluginDescriptor (maybe other plugins might want to use this feature, too..). The third option would be to add it to some component manager that is available, but escaped me while looking at the code.

Any suggestions?

[1] MPLUGIN-1
[2] QDOX-94