Add support for nested comments

I did start implementing this as described in the first comment but decided to do handle nested comments (replies) as GitHub does it.

The benefit of their approach which uses blockquotes instead of nesting is that it will maintain the chronological order of comments/updates which I believe is important for your users to understand the context and latest updates to a post. This is how this implementation looks.

At first glance, it may seem easier with nesting, but think about what happens when you reply to one of many comments in the middle of the comment list. This comment may be relevant for all users and not just the one you replied to, but this comment will be like a needle in a haystack among all other comments.

I believe it will be better if all discussions appear in chronological order and I also believe this will reduce the number of duplicate questions from your users as well as improve the overall quality of the comments.