Package concurrently added to run gulp and preact

Previously we had to run two separate commands to start the gulp preview
server and the actual preact app. This new dev package will run those
commands concurrently under the hood. We just have to use npm run
start.

package.json

@@ -3,7 +3,7 @@
 	"version": "4.2.0",
 	"description": "A blazing fast & offline web playground",
 	"scripts": {
-		"start": "npm run -s dev",
+		"start": "concurrently --kill-others \"gulp start-preview-server\" \"npm run -s dev\"",
 		"build": "preact build --template src/index.html --prerender false",
 		"dev": "preact watch --template src/index.html",
 		"serve-website": "cd packages/website; npm start",
@@ -41,6 +41,7 @@
 		"babel-eslint": "^7.2.3",
 		"babel-minify": "^0.2.0",
 		"babel-plugin-macros": "^2.6.1",
+		"concurrently": "^7.0.0",
 		"eslint": "^4.9.0",
 		"eslint-config-prettier": "^2.3.0",
 		"eslint-config-synacor": "^2.0.2",