Refactor & integrate error reporting with logging

[lagru]

Reporting syntax errors and unknown doctypes is currently using its own implementation instead of relying on a robust logging library. Should probably use a proper logging library, since that should come more robustness and features such as multi-threading support.

Thoughts:

• Python's logging library is probably able to do the job. But I'm wondering if using loguru might get me to the solution faster.

Related:

• Add --group-errors option #30
  The logging library should support delayed grouped logging.