Support filtering parsed objects

[ekohl]

Today JGrep.jgrep parses the input, but in voxpupuli/facterdb#282 I want to further refine results.

As a start I opened #53 so I could understand the API.

One option is to inspect the first argument and only parse it if it's a string, but I'm generally not a fan of those kind of APIs. Factoring it out to a smaller method is probably better.

Before I start I'd like to hear the opinions on this.