[bluesky?] Refactor into modules for each type of dataset

[rclark]

Building modules that each deal with a specific type of file could really help this library be more approachable and maintainable. I'm imagining modules which all have methods like getProjection and getExtent, and a single digest() function that chooses the right module + strings together method calls to build a metadata object.

Most of the functions in the current implementation are like getProjection and while sounding generic, contain logic that branches based on filetype anyways.

Implementing this would make adding new features like different mapnik datasource types for topojson and geojson much more straightforward to implement. For example the mess I made in #67.

cc @GretaCB

[springmeyer]

👍 refactoring this soon hopefully would help unblock #72

[GretaCB]

Refactor PR at #84

[GretaCB]

Merged.