Add .env.example for the project.

[tangoslee]

Currently, MONGO_URL is written hardcoded in the mongoURL.js.

It affects building local development because every developer has a different environment.

This task is doing;

• Add dotenv module
• Make .env.sample and add to git
• Add MONGO_URL environment to .env.sample
• Add .env to .gitignore
• Replace hardcoded MONGO_URL to env value in the mongoURL.js

Example:

• https://www.twilio.com/blog/working-with-environment-variables-in-node-js-html

[kc-leung]

@leroyzhao