feat(post_ermalink): add :second attr option for post permalink

[kkocdko]

What does it do?

Like this:

# _config.yml
permalink: :year/:month/:day/:hour/:minute/:second/:post_title/

Will generate:

example.com/2015/05/06/12/13/14/my-new-post/

How to test

git clone -b BRANCH https://github.com/USER/hexo.git
cd hexo
npm install
npm test

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[SukkaW]

Please add :second for time is omitted in front-matter test case then.

[kkocdko]

I just got an error on the commit title... Now fixed.