Option to output logs to HTML file for easy viewing

Depends on #12. Not exactly the same idea as #3.

We could have an environment variable like `UMBER_HTML_OUT=/some/path.html` to which more detailed logging info could be written (i.e. including timestamps, all logging classes). The HTML could have built-in filtering by class and, if I want to add spans to Umber, I could even have profiling tools like a flamechart etc.