Look for other diff executables

This is RE: #47 and #48. I can add specs, but this will require removing `@@bin` or some other hackery.

Added [`DIFFY_DIFF` environment variable](https://github.com/sshaw/diffy/commit/6e1f1312608af63d978ff4f110d9262427737172#diff-941b6006df0e23b6f8a1f92c70531288R145) based on [your comment](https://github.com/samg/diffy/issues/48#issuecomment-54714248).

Updated the README but not sure what you mean [here](https://github.com/sshaw/diffy/commit/aab3369867d8008db56d2f17cb4d77e68c168d9a#diff-04c6e90faac2675aa89e2176d2eec7d8R46) by *"these two"*.