Adds publish/subscribe pattern to allow plugins and projects to hook into events (#55)

Also adds CLI to ease development.

packages/medusa/src/commands/develop.js

@@ -0,0 +1,45 @@
+import { spawn, execSync } from "child_process"
+import mongoose from "mongoose"
+import chokidar from "chokidar"
+import express from "express"
+import path from "path"
+
+import loaders from "../loaders"
+import Logger from "../loaders/logger"
+
+export default async function({ port, directory }) {
+  const args = process.argv
+  args.shift()
+  args.shift()
+  args.shift()
+
+  execSync("babel src -d dist", {
+    cwd: directory,
+    stdio: ["ignore", process.stdout, process.stderr],
+  })
+
+  let child = spawn("medusa", [`start`, ...args], {
+    cwd: directory,
+    env: process.env,
+    stdio: ["pipe", process.stdout, process.stderr],
+  })
+
+  chokidar.watch(`${directory}/src`).on("change", file => {
+    const f = file.split("src")[1]
+    Logger.info(`${f} changed: restarting...`)
+    child.kill("SIGINT")
+
+    execSync(`babel src -d dist`, {
+      cwd: directory,
+      stdio: ["pipe", process.stdout, process.stderr],
+    })
+
+    Logger.info("Rebuilt")
+
+    child = spawn("medusa", [`start`, ...args], {
+      cwd: directory,
+      env: process.env,
+      stdio: ["pipe", process.stdout, process.stderr],
+    })
+  })
+}