FAQ: Add zapfilter (#939)

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"}

FAQ.md

@@ -157,6 +157,7 @@ We're aware of the following extensions, but haven't used them ourselves:
 | `github.com/fgrosse/zaptest` | Ginkgo |
 | `github.com/blendle/zapdriver` | Stackdriver |
 | `github.com/moul/zapgorm` | Gorm |
+| `github.com/moul/zapfilter` | Advanced filtering rules |
 
 [go-proverbs]: https://go-proverbs.github.io/
 [import-path]: https://golang.org/cmd/go/#hdr-Remote_import_paths