Force POM generation cli option

This CLI option allows the plugin to ignore an existing pom.xml in the root of the project directory and force generation of a new temporary pom for use with maven-install or maven-deploy.

This is very useful if you use maven to control a much larger multi-module project build, of which the grails plugin or application is only one module in a larger project. Our company use maven to drive our product build, but it invoke the grails build for the module via cli. This means that the dependencies for the project are defined entirely within BuildConfig.groovy and not in the "driver" pom.

I realise that very few people will use this option. It is false by default, and is a very simple implementation and no complex logic. Let me know if you'd like it to work differently, or you'd like it documented in the readme.