Add ESLint. Adopted standards

[gavination]

Description

Adds linter. Fixes #6

Checklist for success

• [x] This change isn't already implemented by a pending pull request
• [x] This change is a child commit of a reasonably recent commit in the master branch
  • Requests need not be a single commit, but should be a linear sequence of commits (i.e. no merge commits in your PR)
• [x] All tests pass locally (see the testing section for more info)
• [x] All New behaviors or major functional changes have tests