chore(FAQ): add zapfilter

[moul]

This utility allows creating a logger with "advanced" filters like in the following example.
It is particularly convenient to use with a CLI flag package or using environment variables.

	core := zap.NewExample().Core()
	// *=myns             => any level, myns namespace
    // info,warn:myns.*   => info or warn level, any namespace matching myns.*
	// error=*            => everything with error level
	logger := zap.New(zapfilter.NewFilteringCore(core, zapfilter.MustParseRules("*:myns info,warn:myns.* error:*")))
	defer logger.Sync()

	logger.Debug("top debug")                                 // no match
	logger.Named("myns").Debug("myns debug")                  // matches *:myns
	logger.Named("bar").Debug("bar debug")                    // no match
	logger.Named("myns").Named("foo").Debug("myns.foo debug") // no match

	logger.Info("top info")                                 // no match
	logger.Named("myns").Info("myns info")                  // matches *:myns
	logger.Named("bar").Info("bar info")                    // no match
	logger.Named("myns").Named("foo").Info("myns.foo info") // matches info,warn:myns.*

	logger.Warn("top warn")                                 // no match
	logger.Named("myns").Warn("myns warn")                  // matches *:myns
	logger.Named("bar").Warn("bar warn")                    // no match
	logger.Named("myns").Named("foo").Warn("myns.foo warn") // matches info,warn:myns.*

	logger.Error("top error")                                 // matches error:*
	logger.Named("myns").Error("myns error")                  // matches *:myns and error:*
	logger.Named("bar").Error("bar error")                    // matches error:*
	logger.Named("myns").Named("foo").Error("myns.foo error") // matches error:*

	// Output:
	// {"level":"debug","logger":"myns","msg":"myns debug"}
	// {"level":"info","logger":"myns","msg":"myns info"}
	// {"level":"info","logger":"myns.foo","msg":"myns.foo info"}
	// {"level":"warn","logger":"myns","msg":"myns warn"}
	// {"level":"warn","logger":"myns.foo","msg":"myns.foo warn"}
	// {"level":"error","msg":"top error"}
	// {"level":"error","logger":"myns","msg":"myns error"}
	// {"level":"error","logger":"bar","msg":"bar error"}
	// {"level":"error","logger":"myns.foo","msg":"myns.foo error"}

[codecov]

Codecov Report

Merging #939 (d9e56bb) into master (7b21229) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #939   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files          44       44           
  Lines        1974     1974           
=======================================
  Hits         1940     1940           
  Misses         27       27           
  Partials        7        7           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7b21229...d9e56bb. Read the comment docs.

[abhinav]

Thanks!