support nested aggregations and cardinality (#102, #103)

[willkg]

• Move thing_to_key to utils module
• Add support for nested aggs and cardinality (#102, #103)
• Update README (#102, #103)

The following all work now:

supersearchfacet --_facets=product

supersearchfacet --_facets=product --leftover-count

supersearchfacet --_facets=_cardinality.product

supersearchfacet --_facets=_cardinality.product --leftover-count

supersearchfacet --_aggs.product=platform

supersearchfacet --_aggs.product=platform --leftover-count

supersearchfacet --_aggs.product=release_channel

supersearchfacet --_aggs.product=release_channel --leftover-count

supersearchfacet --_aggs.product.version=_cardinality.install_time

supersearchfacet --_aggs.product.version=_cardinality.install_time --leftover-count

supersearchfacet --_histogram.date=product --_histogram_interval.date=1d

supersearchfacet --_histogram.date=product --_histogram_interval.date=1d --leftover-count

supersearchfacet --_histogram.date=_cardinality.product --relative-range=5d

Fixes #102 Fixes #103