Add option to send raw request body to command

The existing `entire-payload` option sends a JSON representation of the
parsed request body.  Add a new `raw-request-body` source to send the
raw request body.

Fixes #439

webhook_test.go

@@ -572,6 +572,25 @@ env: HOOK_head_commit.timestamp=2013-03-12T08:14:29-07:00
 		`success`,
 		``,
 	},
+	{
+		"txt-raw",
+		"txt-raw",
+		nil,
+		"POST",
+		map[string]string{"Content-Type": "text/plain"},
+		"text/plain",
+		`# FOO
+
+blah
+blah`,
+		false,
+		http.StatusOK,
+		`# FOO
+
+blah
+blah`,
+		``,
+	},
 	{
 		"payload-json-array",
 		"sendgrid",