Replacing duoshuo with GitHub Comments

[7anshuai]

Inspired by http://donw.io/post/github-comments/.

I add a github comments prototype in my blog theme. If nico add a official version, i think it's very useful when using nico to build gh-pages.

[hotoo]

Looks good. PR is welcome.

[7anshuai]

Simply read a github issue id/url from the meta of post, so we can use the id/url to fetch comments in themes.
I will open a PR in the nico one theme as soon as possible.

[7anshuai]

Already created a PR to the one theme.