Add support for dynamic log level in Uinta.Plug

[brettbeatty]

I received a request for 500-level responses to get logged with a level of :error, but Uinta does not currently support log levels that change based on the status code (conn-ditions?). Without thinking through the issue in its entirety I would maybe be of the opinion that meta-logs about requests/responses are always informational and for 500-level responses the reason they're 500-level ought to be logged at the appropriate level. The request, however, does not seem unreasonable, and Phoenix.Logger supports dynamic log level, so I'm proposing it for Uinta.