feat: added support for Disqus

[dwmkerr]

This PR adds support for Disqus. If the standard Hugo disqusShortname configuration is provided, comments will be enabled for pages. This is a non-breaking change, comments are disabled by default.

Apologies if I've got something wrong here, am fairly new to Hugo!

Disqus can be disabled for any content by setting the following in the frontmatter:

disable_comments: true

See effectiive-shell for an example. Disqus is disabled on the main index page, and enabled for each chapter.

[alex-shpak]

Hi! Looks good, thanks for your contribution.
I would suggest few things:

• Add space between footer and Disqus block (maybe 1 or 2 em)
• Rename disable_comments to BookDisqus defaulting to true. To keep it consistent with other params.

Since DisqusShortname already checked here
https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/embedded/templates/disqus.html#L3

It could look like

{{ if default true .Params.BookDisqus }}
  {{ template "_internal/disqus.html" . }}
{{ end }}

[dwmkerr]

Hi @alex-shpak, sorry for the delay, I've been travelling! OK here are the updates:

1. Added padding of 1rem to the disqus thread
2. I've renamed disable_comments to bookDisableComments to keep the parameters consistent.

I don't think I'd fully updated the PR before; there are actually two new configuration fields:

1. disqus_shortname - one of the standard fields from Hugo (https://gohugo.io/templates/internal/#configure-disqus). I've used this rather than bookDisqusShortname as a bit like the Google Analytics config it is one of the standard fields
2. bookDisableComments - this is used in frontmatter only, to disable comments for specific content. For example, effective-shell has Disqus enabled via disqus_shortname, but comments are disabled on the frontpage only.

I think this makes sense; it means nothing should change for existing users, they still need to opt-in for disqus, but they can opt-out of pages if it doesn't make sense to have comments for them. Thanks for the heads up on the spacing too - looks much cleaner now!

BTW if you'd prefer me to make this a single commit, I can open a new PR with these commits squashed.

[alex-shpak]

Good,
I will merge and do some minor changes.

I will add BookComments site config, for global switch, which will have similar frontmatter switch.
To be able to disable by default and enable for some pages only.

[alex-shpak]

Pushed to master.
Global BookDisqus setting and per page bookDisqus frontmatter param

[dwmkerr]

Thanks @alex-shpak, I appreciate the patience as I get familiar with the project!

[alex-shpak]

@dwmkerr Hi!
I made some changes to comments: 4b641b0
Comments got separate partial partials/docs/comments so that alternative commenting engine can be added. Also BookDisqus param is not BookComments

[dwmkerr]

Looks great @alex-shpak, I think it's much cleaner the way you've structured it, really nice to be able to potential switch out comment systems 😄