Make use of flag optional in bind

Expose an additional function, bind.WithFlag(), which allows callers to
use the previously-default "global flag" mode.

This change allows the bind string parser (etc.) to be used without
unwanted side effects. The behavior of the top-level "goji" package has
not been changed.

Fixes #47.

bind/bind.go

@@ -39,7 +39,18 @@ var bind string
 func init() {
 	einhornInit()
 	systemdInit()
+}
 
+// WithFlag adds a standard flag to the global flag instance that allows
+// configuration of the default socket. Users who call Default() must call this
+// function before flags are parsed, for example in an init() block.
+//
+// When selecting the default bind string, this function will examine its
+// environment for hints about what port to bind to, selecting the GOJI_BIND
+// environment variable, Einhorn, systemd, the PORT environment variable, and
+// the port 8000, in order. In most cases, this means that the default behavior
+// of the default socket will be reasonable for use in your circumstance.
+func WithFlag() {
 	defaultBind := ":8000"
 	if bind := os.Getenv("GOJI_BIND"); bind != "" {
 		defaultBind = bind

goji.go

@@ -44,6 +44,10 @@ import (
 	"github.com/zenazn/goji/graceful"
 )
 
+func init() {
+	bind.WithFlag()
+}
+
 // Serve starts Goji using reasonable defaults.
 func Serve() {
 	if !flag.Parsed() {