First draft of a deep glance reader.

TODO:
  - Add some documentation. Document invariant between
    CSV files docviewer and metadata.
  - Hardcoding file names seems bad. How can I better utilize
    the configuration system?
  - Maybe parallelize? Bottleneck doesn't seem to be in generating
    stream items on smaller samples.
Add some extra documentation about the `external_stages_path`
config option.