[closure-compiler-maven-plugin] enable --@define compiler command line option

@jlgrock do you think you can quickly look at this? Comments welcome. One thing I am not sure about is if we need to honour `@define` options in `generateSyncLibrary` method as well.

I am missing tests for now. I was trying to implement test according to <http://maven.apache.org/plugin-testing/maven-plugin-testing-harness/getting-started/index.html> but it did not work for me. Test was still complaining about some missing classes or methods... it would probably need quite careful setup of versions of additional test scoped maven plugin artifacts or something like that. I was unable to get it right.

Also documentation needs to be updated accordingly.

Closes #37