Add reason field to CommentFlag

[rollue]

When a user flags a post, he/she is often given a form to fill out why he/she is flagging it for removal.
I think this is such a common case so it may be worthwhile to add a reason field to the CommentFlag model - and it is especially true if you're developing a mobile app, with it being compulsory on iOS devices to include this feature.
We can name it like reason, description or message ... etc.

[rollue]

If this sounds reasonable, I'll try to add this with a PR :)