Add callbacks support

[poohitan]

Added support for built-in Disqus callbacks.

Motivation and Context

Currently this module support only onNewComment callback, however more callbacks are available in Disqus. This is the full list of callbacks found in Disqus embed.js script:

• preData
• preInit
• onInit
• onReady
• afterRender
• preReset
• onIdentify
• beforeComment
• onNewComment
• onPaginate

After a few tests I've found that actually only onNewComment and onReady are firing, maybe the rest are reserved for the future.

This PR fixes #61.

How Has This Been Tested?

I built the package and tested it on my own website.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.
• [x] All new and existing tests passed.

[tterb]

@poohitan It looks like another Disqus user submitted a discussion with similar findings, but seemed to believe that onPaginate() was firing as well.
https://disqus.com/home/discussion/channel-discussdisqus/api_supported_callbacks/

I can test these this evening and see what I'm able to get working, but I think we'll probably only want to add support for the callbacks that we're able to confirm are working.

[poohitan]

@tterb I saw that discussion as well and tried the onPaginate callback, but with no success, not sure what's the reason. Tested it with 55 comments, 5 of which went to the second page (so I believe onPaginate should have fired when I clicked to load more comments).

Maybe we can document only those which are already working but leave the code as it is in case other callbacks will start working in the future? Anyway, looking forward to your feedback.

[poohitan]

@tterb thanks!