add FilterStack.lexer, fixes andialbrecht/sqlparse#806

[illes]

Thanks for contributing!

Before submitting your pull request please have a look at the following checklist:

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

In addition, please take care to provide a proper description on what your change does, fixes or achieves when submitting the pull request.

[illes]

fixes #806