add async mock capability.. fixes #26

example/mock-async/.local-web-server.json

@@ -0,0 +1,8 @@
+{
+  "mocks": [
+    {
+      "route": "/",
+      "module": "/mocks/delayed.js"
+    }
+  ]
+}

example/mock-async/mocks/delayed.js

@@ -0,0 +1,10 @@
+module.exports = {
+  response: function (ctx) {
+    return new Promise((resolve, reject) => {
+      setTimeout(() => {
+        ctx.body = `<h1>You waited 2s for this</h1>`
+        resolve()
+      }, 2000)
+    })
+  }
+}