Allow using external Shader implementation

Currently if multiple shaded artifacts contain the same fully-qualified class name, the plugin issues a warning on the console like this:

[WARNING] We have a duplicate foo.bar.Baz.class in <path-to-jar-in-local-repo>

In many cases such duplicates are a serious problem, e.g. when they are not the same version of that class.

It would therefore be useful if the plugin (specifically the DefaultShader class) provided the option of failing the build if any duplicates are detected. This would be particularly useful in a continuous integration environment where there's no human operator to check the console output for warnings.