feat(plugin-eslint): implement runner

[matejchalk]

Closes #27

Notes

• implemented and tested runner logic
• runs ESLint via Node API and converts results to audit outputs
• change audit outputs in models to include slug, but not other metadata (already provided in initializer, so unnecessary burden on plugin author)
  • added logic to core which looks up metadata by audit slug to create reports
• since ESLint results only include failed rules, all audits slugs have to be passed to runner so that it can report passed audits
  • might also be necessary for other "linter" plugins (Stylelint, TypeScript), in which case the core could provide a built-in solution for it 🤔
• additional entry point added for executable bin.js file
  

Next steps

• E2E test for ESLint plugin #100

[BioPhoton]

Looking forward to get this merged!