artifacts mojo: allow to exclude extensions

[kwin]

Similar to excluding/including classifiers added in #100 there should be the possibility to include/exclude certain extension.

My use case is ASC files being generated by https://maven.apache.org/plugins/maven-gpg-plugin/sign-mojo.html. It uses the same phase by default (verify) and if it is executed prior checksum-maven-plugin:artifacts the asc get additional checksums as well.
Executing it afterwards leads to generating signature files for the attached checksums as well which is usually not desired either.