feat: support IDN

[curbengh]

Based on hexojs/hexo-generator-feed#82, related issue hexojs/hexo-generator-feed#63

[coveralls]

Coverage remained the same at 100.0% when pulling d84c404 on curbengh:idn into ef8a8cf on hexojs:master.

[tomap]

I will test it on a sample site and merge :)

[tomap]

I tested with a site without IDN, but in that case, https://hexo-theme-minidyne-demo.netlify.com/sitemap.xml
it seems to work

[curbengh]

You can test locally and change the url.
My local test yields sitemap.xml

---

I just noticed this PR doesn't uriencode root value, even though it should. I'll update.
hexo-generator-feed may need to be updated as well.

---

URL api can be used to parse the url to separate each component. Legacy api is needed for node 8 compatibility. It doesn't encode uri however.

const url = require('url')
const string = 'https://日本語テストgôg.com/hexô-testing'

console.log(url.parse(string).href)
// https://xn--gg-8ja8200c3ban0926g0ecl32k.com/hexô-testing

console.log(url.format({
  protocol: url.parse(string).protocol,
  hostname: url.parse(string).hostname,
  pathname: encodeURIComponent(url.parse(string).pathname),
}))

// https://xn--gg-8ja8200c3ban0926g0ecl32k.com/hex%C3%B4-testing

URL api always output hostname in punycode though. is punycode okay?

[curbengh]

Superseded by #66