Add ability to leave replies to existing comments

This PR addresses the issue in  #17 
We can separate comments in 2 categories: **Comment** and **Reply**. 

Every **Reply** is a **Comment**, but the inverse is not true.

Also, a **Reply cannot** have replies and must belong to a **Comment**, which may have from 0 to N replies. ( IMHO, using the "side-comments" format, makes no sense to have more than 1 level of nesting, that's why a **Reply** can't be replied, but this can be discussed );

A **Reply, and a **Comment** with no replies can be normally deleted and removed from the list, however, a **Comment** with at least one **Reply**, can be deleted, but **cannot** be removed. To do so, I've used the flag `deleted`, that will display the message "_Comment deleted by the author_". This was the trickier part to solve for me and it's entirely open to discussion.

Cheers ;)