Improve usability

[cthoyt]

This PR does two things:

1. It adds three utility functions to make it easier to load corpus lists from arbitrary data structures (instead of relying on internal state of a file path):

• pubtator_loader.from_lines for an arbitrary iterable of strings
• pubtator_loader.from_path for any path or path-like object
• pubtator_loader.from_gz for a path or path-like object pointing to a file that needs to be gunzipped.

It also updates the example in the README to reflect new usage.

2. Updates imports and type annotations for spacy in pubtator_document.py so users can use this code if spacy isn't installed.

If you're able to merge this, it would be great to make a new release too so I could clean up the dependencies in my code. Thanks for making a nice library for us!