This change implements a color spectrum for severity levels, disabled by default.

Add a color spectrum to lager to support ANSI colors enabled in R16A
The default is {colored, false} for backwards compatibility.
When set to true the configurable colors are preset to the following:
- gray(debug), white(info), cyan(notice), yellow(warning), red(error), 
- magenta(critical), white-on-blue(alert), white-on-red(emergency).