… documents from a GCS directory.

Fixes #214

- Note: `from_gcs()` takes in a GCS directory, but it only works for a single sharded document from a single input document source.
- In a GA release, it would be a better practice to have `from_gcs()` take in any GCS directory and output a list of Wrapped Documents. But this would be a backwards-incompatible change now.
  - Not sure if it's possible/advisable to have two possible return types for `from_gcs()` and just have it return a list when there are multiple Wrapped Documents?

- Input data doesn't match what the actual `storage.list_blobs()` method outputs.
- Updated output in `print_gcs_document_tree()` to match the test cases, as this was the intended structure.