feat(middleware): add helpers for authorization middleware

Closes #1 (closed)

src/helpers.js

+const { rule } = require('graphql-shield')
+
+const isAuthenticated = rule()(async (parent, args, ctx, info) => {
+  return !!ctx.user
+})
+
+const isAdmin = rule()(
+  async (parent, args, { user: userId, connectors: { User } }, info) => {
+    if (!userId) {
+      return false
+    }
+
+    const user = await User.model.findById(userId)
+    return user.admin
+  },
+)
+
+module.exports = {
+  isAuthenticated,
+  isAdmin,
+}