Memory Provider: subquery and savedSearch

Provides a Memory Provider example of a server side subquery and savedSearch type

Memory Provider: Add savedSearch type
Memory Provider: Add subquery type
Memory Provider: Facet now coerces values to strings (since number options are strings anyway)
Added facet export data strategy

Notes:

The facet strategy is on this PR but should be moved to contexture-export
Using facet for this could be a little wasteful because it'll get counts in addition to just the terms values.
subquery here does not send it's filter results back to the client to interim caching, which means server-side subquery is currently less efficient then client side for interactive searches
~Once this is merged, we should give contexture-elasticsearch and contexture-mongo a similar treatment, as well as providing something in contexture-react.~ subquery and savedSearch are now implemented without being explicitly tied to any provider

	Warnings
:warning:	:exclamation: This PR is BIG (+436 -116) Please keep it below 500 net changes
:warning:	You should add at least 1 more reviewer to the PR
:warning:	The README has not been updated. Please update the README.

futil-js / contexture-core