Mapper

[umputun]

Add mapping func support

Elements of the output can be altered with a set of user defined function passed as lgr.Map options. Such a mapper changes
the value of an element (i.e. timestamp, level, message, caller) and has separate functions for each level. Note: both level
and messages elements handled by the same function for a given level.

A typical use-case is to produce colorful output with a user-define colorization library.

example with fatih/color:

	colorizer := lgr.Mapper{
		ErrorFunc:  func(s string) string { return color.New(color.FgHiRed).Sprint(s) },
		WarnFunc:   func(s string) string { return color.New(color.FgHiYellow).Sprint(s) },
		InfoFunc:   func(s string) string { return color.New(color.FgHiWhite).Sprint(s) },
		DebugFunc:  func(s string) string { return color.New(color.FgWhite).Sprint(s) },
		CallerFunc: func(s string) string { return color.New(color.FgBlue).Sprint(s) },
		TimeFunc:   func(s string) string { return color.New(color.FgCyan).Sprint(s) },
	}

	logOpts := []lgr.Option{lgr.Msec, lgr.LevelBraces, lgr.Map(colorizer)}