Add serve.js for visual test

test/serve.js

@@ -0,0 +1,12 @@
+const http = require('http')
+const badgen = require('..')
+
+// @example
+// http://localhost:3000/npm/v1.2.3
+const serveBadge = (req, res) => {
+  const [ subject, status, color ] = req.url.split('/').filter(Boolean)
+  res.writeHead(200, { 'Content-Type': 'image/svg+xml;charset=utf-8' })
+  res.end(badgen({subject, status, color}))
+}
+
+http.createServer(serveBadge).listen(3000)