SCHEMA-260. Add NamedLookup and NamedProvider.

tested w/ mvn verify. Adds new unit tests.

I used this pattern in the Kiji CLI tools and now I'm using it again in the DDL shell plugin system (the user loads plugins by name). I think this pattern is generalizable and should be internalized in the delegation library.

This will be required before DDLSHELL-25 can be pushed.