fs.mkdir(sync) supports recursive option

[segayuu]

fs.mkdir(Sync) has implemented an option to execute recursively when the node version is v10.12.0 or later.
This eliminates the need for recursion in hexo-fs.

[coveralls]

Coverage increased (+0.1%) to 96.466% when pulling 6cba1a9 on segayuu:fs.mkdir(Sync)-supports-recursive-option into 336747f on hexojs:master.

[SukkaW]

So it means this PR is head for next major version of Hexo (which will require Node.js 10 or later), right?

[segayuu]

Yes.