Automated codestyle checks

[nezed]

To make it easier to make PR and keep codestyle consistent i propose to add:

• eslint with preferred confg
• precommit hook that checks codestyle (i recommend husky+lint-staged)
• CI checks (may be implemented by running eslint on travis-ci)

One of the most good and not too strict config which i respect is eslint-config-standard
https://standardjs.com/#standardjs--the-rules

Introducing hard codestyle convensions requires mostly automatic code refactoring which will produce large but dummy diff at one time.

@apla i'l make a PR, but i want to hear your opinion about all described above.