Add logging and expose debug output settings in plugin configuration

[PamelaM]

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Many of the issues that have been created are configuration and environment related, so they have been difficult to debug and identify.

This PR exposes the 2 existing debugging flags SHOW_TRACING and SHOW_PARSING to the config file options (as per coverage.py documentation). It also adds 2 more: SHOW_STARTUP and LOG_FILE.

SHOW_STARTUP enables extra logging in code that's called during startup, as well as logging the python and django versions.

LOG_FILE (called log_file_path in the config option) is an optional argument allowing the user to capture all django_coverage_plugin logging to one file, which can be included or requested when tracking down issues.