Merge multiple composable loggers (opposite of Demux)

[oxinabox]

We can imaging Demuxing logs,
into many "streams" which go through a varitety of filters,
but then we might want to merge them back together for a final output, to a common sink.
So we need a MuxLogger

Better name needed.
Also better name needed for DemuxLogger

[oxinabox]

Wait this is not needed.
Composable loggers take destinations,
not sources.

So to mux one just reuses the destination.

eg.

sink = ConsoleLogger(stderr, BelowMinLevel)
filter_a = EarlyFilteredLogger(sink) do log
    log._module==Main || log.level > Logging.Debug
end
filter_b = EarlyFilteredLogger(sink) do log
    log.level > Logging.Info
end

demux = Demux(filter_a, filter_b)

So this is a Documentation issue