Adds a visualize_spans function for using the span visualization in displaCy.

I made a few choices that may need justification:

• Even though spans_key is part of the options passed to displaCy, it feels critical enough to pass as an argument to visualize_spans rather than as a key/value in a dict to displacy_options - but we can obviously undo this.
• I didn't include the span viz in the "display everything" visualize function because none of the base models will have labeled spans in them, so it wouldn't display anything useful.
• Compared to visualize_ner:
  • I removed the manual argument because there's no such option for that with displaCy for spans
  • There's no label filtering/selection with spans, so all labels will be displayed for spans within a span_key. This removes the keys and labels args.

Extra:

• Also added a missing docstring for the show_table argument on visualize_ner (as well as visualize_spans)

Open questions:

• Is there a script to generate the nice readme.md documentation so we can include the new function?
• I'm not totally sure what needs to happen for a release, but I'm happy to help do whatever needs to be done!