Add linter

[simondereuver]

Add a linter to project.

EDIT: Linters have been chosen.

• [x] Create ESLint configuration file.
• [x] Create Styleint configuration file.
• [x] Create HTMLHint configuration file.
• [x] Create separate ESLint configuration file testing enviroment (Mocha).

[simondereuver]

Chose ESLint to project for javascript, Stylelint for css and HTMLHint for html.

Needs to be added to README info: Linter for server-side: npm install eslint eslint-plugin-node eslint-plugin-express --save-dev Linter for client-side: npm install eslint eslint-plugin-html --save-dev Linter for CSS: npm install stylelint stylelint-config-standard --save-dev Linter for HTML: npm install htmlhint --save-dev

[simondereuver]

Done in commit: 6095b81aa517eae29cc8908db4054cb2e3117afa