Added 'accept an answer' comment

Added 'accept an answer' comment. It `@` tags the OP. It has a `[type here]` area where you can put "my", "a", "this" or another word in order to allow it to as a comment on the question or an answer.

Improvements welcome.

Example:

> @OP_USERNAME: If this answer solved your problem, then you should accept it by clicking the checkmark on the answer. This will mark your question as answered, see more on accepting at the [help center](http://askubuntu.com/help/accepted-answer).