Allow static preview to be toggled

This commit refactors the launch command to let the user
toggle static preview on and off respectively.
Add visual feedback on current state of live preview
Refactor _handleStaticPreview

  - renamed to _toggleStaticPreview
  - extracted launching and closing of the server into their own methods (_closeServer, _launchServer)
Ignore node_modules/
Implement static file serving using express

Added two dependencies:
  - Express: https://www.npmjs.org/package/express
  - serve-static (middleware): https://www.npmjs.org/package/serve-static
Updated README
Close server when the current project closes