Allow code coverage to more than /lib dir

Currently lab only provides code coverage through blanket to cwd/lib. Although I know you don't plan on making Lab a test framework for general consumption, allowing flexible folder and file structure is a good thing.

With my changes, it removes the requirement of using the lib folder, and does a negative lookahead check for node_modules and test directories. I have tested the changes with Hapi and Catbox, both of which perform as expected.

Thanks for your consideration.