Memberlist logger should be exported

[cousine]

Hi there,

I think the logger should be exported instead of the LogOutput in the configuration; I'm using a custom logger with a custom format however I must hack a custom io.Writer to have memberlist use my custom logger.

[ryanuber]

Hey @cousine, seems reasonable to use a custom logger. Instead of exporting it directly in the struct, maybe we could do like a CreateLogger(*Config, *log.Logger) analogous to the Create method or something to that effect.

[cousine]

Yeah that would be very helpful 👍 but I'd call it CreateWithLogger tbh 😄

I'll see if I can make a PR for that change

[JensRantil]

See also #76.