Refactor multicast setup (isolated config, etc)

src/multicast/options.go

@@ -0,0 +1,28 @@
+package multicast
+
+import "regexp"
+
+func (m *Multicast) _applyOption(opt SetupOption) {
+	switch v := opt.(type) {
+	case MulticastInterface:
+		m.config._interfaces[v] = struct{}{}
+	case GroupAddress:
+		m.config._groupAddr = v
+	}
+}
+
+type SetupOption interface {
+	isSetupOption()
+}
+
+type MulticastInterface struct {
+	Regex  *regexp.Regexp
+	Beacon bool
+	Listen bool
+	Port   uint16
+}
+
+type GroupAddress string
+
+func (a MulticastInterface) isSetupOption() {}
+func (a GroupAddress) isSetupOption()       {}