#828 Add --keywords option to inspect command

Closes #227

Adds a --keywords flag to inspect that counts keyword occurrences in a schema grouped by vocabulary, based on the approach in https://github.com/sourcemeta-research/jsonschema-keywords

jsonschema inspect schema.json --keywords

Uses SchemaIterator to walk the schema and count keywords per vocabulary. Made it conflict with --json since I wasn't sure how to combine keyword stats with the existing json output format, let me know if you had something else in mind.

Added tests for the normal case and the --json conflict, and updated the docs.