Fixes lwsjs/local-web-server#115

but I'm not really sure if this is the best way to achieve it.

What this PR does?

• Moved the reading stored config function to util file
• Used that util file in both index.js and lib/command/serve.js
• In index.js, use deepmerge to merge the hardcoded init options, the options from listen() argument, and the options from config file
• Added test: test/lws.js