Generate pmd.xml, but skip HTML generation

### New feature, improvement proposal

I would like an option to keep generating the XML file, but to skip the HTML reports.

The `pmd.xml` file is used by Sonar so we don't need the HTML reports in this case.

I've searched on the Internet and looked at the code, but I don't think it's currently possible to skip the Doxia HTML generation.

I guess this is a similar question/request: https://stackoverflow.com/questions/42693443/why-maven-pmd-plugin-generate-pmd-html