Allow to give an explicit directory as source for META-INF/vault

Currently the META-INF/vault structure within the packages is built from <workDirectory>/META-INF/vault.

This directory is used for two purposes

1. It receives the generated filter.xml, the merged properties.xml, the config.xml and the settings.xml
2. It acts as a source for the resulting package's META-INF/vault

Not all meta files below META-INF/vault are being generated through the plugin itself. Some need to be written manually to workDirectory with the help of the maven-resource-plugin.

It would be good to allow to give an explicit source directory for the META-INF/vault directory (similar to jcrSourceDirectory) separated from workDirectory. This would allow to e.g. specify the packages thumbnail image, without being forced to copy it over to from src to target.
In addition this parameter should clearly define which meta files are always generated by the filevault-package-maven-plugin and are therefore never considered, even if they are located in the given META-INF/vault source directory.