Add npm start

[gemmaleigh]

What problem does the pull request solve?

Ensure that 'npm start' can be used to start the govuk_elements application.

If no "start" property is specified on the "scripts" object, it will
run node server.js.

Screenshots (if appropriate):

What type of change is it?

• New feature (non-breaking change which adds functionality)

[NickColley]

Could we change the scripts in package.json to look like

  "scripts":{
    "test": "standard && grunt test",
    "start": "grunt"
  },

(package.json can get the binary globals from the local node_modules so you don't need to hard code them)

Then remove start.js?

[NickColley]

Then we could update https://github.com/alphagov/govuk_elements/blob/master/README.md#running-this-site-locally to use npm start