Leave IDN intact in permalink

[tomap]

Because IDN only contain unicode, they do not need to be punycode encoded.
(I bought one, I want the nice version, not the ugly one)
See https://nodejs.org/docs/latest/api/url.html#url_url_format_url_options

[coveralls]

Coverage increased (+0.007%) to 95.432% when pulling e14f8e0 on tomap:IDNUnicode into 4f07d2b on hexojs:master.

[SukkaW]

Maybe we should implement two encode_url() method for different usages.

[tomap]

I'm not sure there is a usage where IDN needs to be punycode encoded.

[curbengh]

url.format() doesn't support port number in Node 8, see #114. This explains the failed test in Node 8.

[curbengh]

A workaround is to use punycode api

const { toUnicode } = require('./punycode');
return parsed.toString().replace(parsed.hostname, toUnicode(parsed.hostname));

[curbengh]

readme also needs to be updated.

[tomap]

node 8 EOL is arround the corner, and will probably imply that this package, as well as many other and also hexo have a major version bump.
So this PR can wait a bit for another PR to drop node 8 compatibility

[curbengh]

we can use the workaround temporarily until node 8 EOL. I don't think this is a breaking change, I do agree IDN users would prefer not to punycode.

If this PR wait for hexo's major version bump, it would be many months away.

[curbengh]

Added the workaround. another change introduced by this PR is that IDN whether it's in unicode or punycoded is always encoded to unicode.

encodeURL(http://xn--br-mia.com/baz')
// http://bár.com/baz

[curbengh]

I'm not sure there is a usage where IDN needs to be punycode encoded.

When permalink is encoded by default, it breaks encodeURI() (permalink is encoded twice).

If a plugin wants to remain compatible with <v4, they can use encodeURI(decodeURI()) or encodeURL() instead.

If the IDN is not punycoded, encodeURI(decodeURI()) will percent-encode the domain.

But of course, as we learned from hexo-generator-feed & hexo-generator-sitemap, encodeURI()/encodeURI(decodeURI()) should be avoided and use encodeURL() instead.

Note that I still +1 this PR.