token source position tracking

[dylanscott]

This PR adds tracking of token source positions, accessible with new pos and length attributes on the Token class and its subclasses. As mentioned in the corresponding issue (#793), we've been using a forked version of sqlparse with this addition to power syntax highlighting in a production system for awhile, and were hoping to upstream the change if it's acceptable.

• [x] ran the tests (pytest)
• [x] all style issues addressed (flake8)
• [x] your changes are covered by tests
• [ ] your changes are documented, if needed