Allow users to override the default logger and handler [Feature Request]

[ethagnawl]

I came across unexpected behavior when attaching Dash instances to a Flask application. The logger attached to the Dash app causes log messages sent to my application's logger (e.g. current_app.logger.info('foo')) to be output N+1 times (N=number of Dash applications attached to Flask app).

I was able to work around this by removing the handler added by Dash:

my_dash = dash.Dash(__name__,
    server=app,
    url_base_pathname='/my_dash/',
    assets_folder="../dashboards/foo/bar/assets"
)

if (my_dash.logger.hasHandlers()):
    my_dash.logger.handlers.clear()

Ideally, I'd either be able to opt out of having Dash attach the handler and/or provide a custom logger/handler via a Dash constructor kwarg. I'd be happy to take a pass at submitting a PR if this is something the maintainers would consider.