Create generic interfaces for swappable components

[lintool]

Would be nice to demonstrate the generality of our platform. Couple obvious ideas come to mind:

• Generic doc store "source" interface, swappable between Solr and ES.
• Generic graph store "sink" interface, swappable between Neo4j and some other graph store (Jena?)

[ryan-clancy]

See https://github.com/dstlry/dstlr/tree/datasource for a start