Document gathering custom data with the local datagatherer

Currently Preflight has a set of built-in datagatherers. This is a limitation, because if someone wants to create a new package but doesn't have a datagatherer that provides the information for it, the only option is to write Go code and create a new datagatherer. This might not be ideal for some people.

Ideally, there should be a way of plug into Preflight any external data provided from an arbitrary process or file or whatever that is generic.