chore: require at least node 8

[curbengh]

• add support for ES2019 (through eslint-plugin-node@9, see next comment)
  • Node 8 only partially supports ES2018 and ES2019, node 8 in travis (of other repos that uses this) should fail when using unsupported es2018/es2019 features.
• update to eslint-plugin-node@9 to support node 12. It still supports eslint@5, but requires >=8.10.0 which is higher than hexo's (Drop NodeJs 6 support & Require NodeJs >= 8.6 hexo#3508 (comment)). eslint-plugin-node@8 may be safer.
• ignore nyc logs and package-lock. "coverage/" is used by the deprecated istanbul.

[curbengh]

The config eslint.js may requires some changes.

[curbengh]

just noticed eslint-plugin-node@9 defaults to ecmaversion 2019, so that value is not needed here. Tested with hexojs/hexo-generator-sitemap#46.

{ "env": { "es6": true } } is still required for new ES6 global variables.