Add support for '--flag arg'

flag.go

diff --git a/flag.go b/flag.go
index 55c381c..865259b 100644
--- a/flag.go
+++ b/flag.go
@@ -585,6 +585,10 @@
 	} else if len(flag.NoOptDefVal) > 0 {
 		// '--flag' (arg was optional)
 		value = flag.NoOptDefVal
+	} else if len(a) > 0 {
+		// '--flag arg'
+		value = a[0]
+		a = a[1:]
 	} else {
 		// '--flag' (arg was required)
 		err = f.failf("flag needs an argument: %s", s)

flag_test.go

diff --git a/flag_test.go b/flag_test.go
index 67a6a27..a5928e5 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -176,7 +176,7 @@
 		"--int8=-8",
 		"--int32=-32",
 		"--int64=0x23",
-		"--uint=24",
+		"--uint", "24",
 		"--uint8=8",
 		"--uint16=16",
 		"--uint32=32",