fix(isExternal): url parameter can be an internal link

[curbengh]

I just noticed a silent issue.

const data = urlObj(url);
const host = data.hostname;

data will be a string when url is an internal link, resulting in host being assigned as undefined. when it is "undefined", it won't be used, thanks to

if (!sitehost || typeof data === 'string') return false;

While current code won't cause any crash, it's a bit unsafe to leave it "undefined" and also could lead to incorrect assumption that data is always a URL object.

[coveralls]

Coverage remained the same at 95.455% when pulling 9dfc53d on curbengh:host-object into 71bbcc4 on hexojs:master.

[SukkaW]

Might have a look at hexojs/hexo#3839 ? In this PR I bring up a poc on performance. In that PR I also fix this issue by moving const host = data.hostname;.

[curbengh]

#124