Implement #22 from upstream

I refactored a bit of the jgrep runtime logic to make adding additional options somewhat easier. This also adds the --slice option for pulling out a subset of documents based on document index.

On a side note, there should probably be tests for the jgrep runtime, or the functionality from the runtime should be rolled into the library. I did a bunch of manual testing to verify things still worked, but since my changes didn't touch the library itself, there are no unit tests.