Listen on IPv4 and IPv6 by default

As noted [here](golang/go#9334 (comment)), the "correct way" to listen on both IPv4 and IPv6 loopback interfaces is to omit the address part.

Closes: #2

And will also close: wincent/vim-clipper#1

README.md

@@ -158,9 +158,9 @@ As previously noted, Clipper supports a number of command line options, which yo
 ```
 Usage of ./clipper:
   -a string
-        address to bind to (shorthand) (default "127.0.0.1")
+        address to bind to (shorthand)
   -address string
-        address to bind to (default "127.0.0.1")
+        address to bind to
   -c string
         path to (JSON) config file (shorthand) (default "~/.clipper.json")
   -config string
@@ -442,6 +442,8 @@ THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 ## master (not yet released)
 
 - Linux support via `xclip` instead of `pbcopy` (patch from Nelson Fernandez).
+- Added `--executable` and `--flags` options.
+- On dual-stack systems, listen on both IPv4 and IPv6 loopback interfaces by default.
 
 ## 0.3 (3 June 2016)
 

clipper.go

@@ -86,7 +86,7 @@ func initFlags() {
 }
 
 func setDefaults() {
-	defaults.Address = "127.0.0.1"
+	defaults.Address = "" // IPv4/IPv6 loopback.
 	defaults.Port = 8377
 
 	if runtime.GOOS == "linux" {
@@ -224,11 +224,15 @@ func main() {
 		addr = settings.Address
 	}
 	if strings.HasPrefix(addr, "/") {
-		log.Print("Starting UNIX domain socket server at ", addr)
 		listenType = "unix"
+		log.Print("Starting UNIX domain socket server at ", addr)
 	} else {
-		log.Print("Starting TCP server on ", addr)
 		listenType = "tcp"
+		if addr == "" {
+			log.Print("Starting TCP server on loopback interface")
+		} else {
+			log.Print("Starting TCP server on ", addr)
+		}
 		addr = fmt.Sprintf("%s:%d", settings.Address, settings.Port)
 	}
 	listener, err := net.Listen(listenType, addr)