Compliance levels

The compliance levels reported on the [libraries page](https://github.com/jmespath/jmespath.site/blob/master/docs/libraries.rst) should be a bit more granular; the specification is changing over time and when it's updated libraries are no longer "fully compliant" (e.g. at the present point in time the Ruby library is lacking proper support for "and" and "or" expressions from JEP 9, so shouldn't really be listed as fully compliant).

Perhaps the specification could have versions instead and the libraries could instead indicate the version that they're compliant with? This would give users an indication of the features that they can expect to work and the features that they could expect not to.