Modernization/ESNext code

[jayands]

I've already started to address this in my PR and stuff, but this project needs to be brought up to ${currentYear || 2018} standards, especially since the author has tried doing so with Babel 7 already.

Some things to consider:

• [x] Use Stage 3 or lower (suggestion: stage 0) features in code
• [x] Use @babel/preset-env for transpiling
• [x] Dropping support for EOL Node.js, like v7 (as Node themselves have: cf. https://github.com/nodejs/Release#lts_schedule)
• [x] Prettier for code style consistency with minimal configuration
• [x] Linting (suggestion: eslint/recommended) for catching errors
• [x] Lodash for ensuring that modern things like map and filter work appropriately
• [x] Rollup (#4)