Implement ignore_fields for not comparing certain data.

Despite being documented, ignore_fields was not actually being used to
avoid testing data fields (containing e.g. timing output) which the user
might want in the output but does not want to compare/validate.  Oops...

Caught during reviewing f6bfba6afcd9342b8f2c19d056ad16a8d79a0dd9.
Acknowledge contributions and suggestions in README.