Add ServeWithOptions

This adds support for options to be added to 'Serve' and the app struct.
Options are implemented following the 'functional options' pattern
(https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis and
https://commandcenter.blogspot.co.uk/2014/01/self-referential-functions-and-design.html).

Future options can be added by creating an exported func that returns a closure
modifying the app struct, like the following:

    func HaltAndCatchFire(literallyCatchFire bool) option {
        return func(a *app) {
            a.haltAndCatchFire = literallyCatchFire
        }
    }

then in user code:

    gracehttp.ServeWithOptions(
        []*http.Server{ &myServer },
        gracehttp.HaltAndCatchFire(true),
    )

gracehttp/http.go

@@ -24,6 +24,8 @@ var (
 	ppid       = os.Getppid()
 )
 
+type option func(*app)
+
 // An app contains one or more servers and associated configuration.
 type app struct {
 	servers   []*http.Server
@@ -117,11 +119,7 @@ func (a *app) signalHandler(wg *sync.WaitGroup) {
 	}
 }
 
-// Serve will serve the given http.Servers and will monitor for signals
-// allowing for graceful termination (SIGTERM) or restart (SIGUSR2).
-func Serve(servers ...*http.Server) error {
-	a := newApp(servers)
-
+func (a *app) run() error {
 	// Acquire Listeners
 	if err := a.listen(); err != nil {
 		return err
@@ -172,6 +170,23 @@ func Serve(servers ...*http.Server) error {
 	}
 }
 
+// ServeWithOptions does the same as Serve, but takes a set of options to
+// configure the app struct.
+func ServeWithOptions(servers []*http.Server, options ...option) error {
+	a := newApp(servers)
+	for _, opt := range options {
+		opt(a)
+	}
+	return a.run()
+}
+
+// Serve will serve the given http.Servers and will monitor for signals
+// allowing for graceful termination (SIGTERM) or restart (SIGUSR2).
+func Serve(servers ...*http.Server) error {
+	a := newApp(servers)
+	return a.run()
+}
+
 // Used for pretty printing addresses.
 func pprintAddr(listeners []net.Listener) []byte {
 	var out bytes.Buffer