feat: add 'disqus' comment feature to each page

[singleye]

Hi,

I use docdock for some time, and I miss the comment feature like 'disqus' which is useful for document sharing. This pull request adds the 'disqus' commenting feature.

It works this way:

1. To enable the feature, add ''disqusShortname' into 'config.toml' or each page's param field. Empty or missing such config would disable the feature.
2. Each page has higher priority than global config. And it could be specified a different short name consider that single page is created by another people in the team.
3. It falls back to the global config.toml if the page's config is missing.

I've tested with both original/flex themes.

Thanks,

[vjeantet]

Hello, i'm ready to merge your good PR !

Why mathjax ?

[Pilskalns]

Hi @singleye,

Could you clear up this PR to have only Disqus commit?