Improve debugging and tracing in Logger.

* Change setting of debugging flags to support handler-specific flags
  from command line (e.g., "debug pre").

* Begin to phase out generic debugging option by removing references
  to env.conf.parsoid.debug throughout the codebase.

* Setting --debug flag automatically enables --trace flag.

* Set up a default tracing / debugging backend in Logger, with
  formatting specific to tracing / debugging events.

* Create separate regexp tests that are used to register tracing,
  debugging, and default backends. For example, the regexp for the
  default tracer is based on command-line tracing flags. If the
  command is "--trace pre", the default tracer should be registered
  with "/trace/pre(\\/|$)/".

* Create an all-purpose regexp that tests for any relevant logType,
  whether set on env by default or passed in from the command line.

* TODO: Have to improve tracing output. It still looks like it does
  in the gist: https://gist.github.com/mariapacana/8242e098cc0c32b9b4f2

* TODO: Figure out whether registerBackend should be updating the
  testAllRE whenever a new backend is added.

Change-Id: I9d703676e911bf62f3c9cbfb46ea0ff3ad300072
Merge "Improve debugging and tracing in Logger."