refactor: whatwg url api

[curbengh]

legacy URL api has been deprecated in Node v11; while the API is still available in Node 12, eslint-plugin-node considers it as a deprecated api, thus will emit error if detected.

WHATWG url is actually introduced back in Node v7, thus contrary to my previous PR (hexojs/eslint-config-hexo#19), it is supported in Node 8. The only thing is that Node 8 requires const {URL} = require('url'), whereas newer Node doesn't.

Another noticeable difference is that WHATWG remove the port number if it corresponds to the protocol (e.g. http:80, https:443), which explains the change of port number in the unit test.

decode_url is not refactored to the newer API for now due to a bug in Node 8,

const {format} = require('url')
const bob = new URL('http://user:pass@foo.com:8080/')
console.log(format(bob, { unicode: true }))
// http://user:pass@/
// Domain is removed if port number exists

Once we drop Node 8, not only we can update decode_url, we can also remove const {URL} = require('url') line. It's ok to retain it, but it's not necessary.

[coveralls]

Coverage increased (+0.08%) to 95.278% when pulling f60127b on curbengh:whatwg-url into b292332 on hexojs:master.

[SukkaW]

LGTM!

[curbengh]

Added a test for 4964d80.