Update Extractor and Loader Pages so they're built from YAML definition

Currently, the existing Extractor and Loader pages are built from markdown files. We should be able to dynamically generate them based on the YAML definition.

This would require adding a few new sections to the YAML definition. I think it would make sense to be able to import the clean Singer spec #37 (moved) to make this easier.

FYI @pnadolny13