Prefer README.md and README to other README.*

This will prefer `README.md` and `README` (the former taking higher precedence) to any other `README.*` files. Note that there is no test case, as the current means of testing this uses the project root; if you would like me to change where the basic.js test case reads from, I can add a test case.

All existing tests continue to succeed.

I attempted to follow the existing coding style the best I could discern.