Allow for client-specified name for Assertions class

[dancerjohn]

The generated entry-point class is called Assertions. If I am using the std Assertions class and my generated Assertions class in the same test, I can only import one which means the usage of the other has to be fully qualified. It would be nice to be able to add a prefix so that the generated entry-point classes are name MyProjectAssertsion, MyProjectJUnitAssertions, etc.

[dancerjohn]

This would require an update AssertionsEntryPointGenerator.generateAssertionsEntryPointClassFor in assertj-assertions-generator to take an optional file name prefix. My question is, is it OK to change the method signature if I update the plugin or would an overload be better?

If this suggestion seems like a good idea, I will take a stab at it.