Add functionality to set Log level

[Keridos]

Currently on 0.11.0 the messages about blocked requests are treated as error logs so they spam in my traefik logs. Could you please add in a functionality to set the log level in the middleware?

[david-garcia-garcia]

Probablye related to #92 the middlewares do not have yet direct access to traefik's logger:

traefik/traefik#8204

[david-garcia-garcia]

@Keridos I've made a PR where you can customize how the middleware logs:

#96

You can either send the middleware logs to their own target so they don't mix with traefik's logs, or adjust the log level to something below INFO (warn, error) which is now the level used for blocked requests.