Allow the working directory to be specified.

The maven-karma-plugin will fail to run karma if the karma execution is configured to happen in a submodule.  This is because maven tries to invoke karma at the top-level directory, where node_modules does not exist.  node_modules only exists in the sub-directory, where the submodule is defined.

I've fixed this by setting the working directory of `builder` to be the directory of the submodule.  By default, this is ${basedir}, but it can be overrided in config.  

Thanks!

Ken