Support configuration of base severity level #1740

Currently there is no way to modify base severity level of a LoggingHandler. Logs with the same severity with the base could be more efficiently sent to Stackdriver.

Some ideas:

1. Do nothing.
   The level the handler is set to is probably the most chatty anyway.
2. Give it its own config param.
3. Create setBaseLevel method.
   This requires us to make bundling a little more resilient first.