Add #isTv() to properly detect common TVs

Hi !

First, thanks for this great library!

TV detection is important to us, so I've implemented it, keeping the same detection patterns as for mobile and tablets (btw, the architecture makes it very easy to add new detections :+1:).

I know there's already a discussion about this (#203) but we needed this *now* so I went ahead and added it to fulfil our needs. :)

Are you interested in this addition? If yes, please let me know if there's anything I can tweak to make this PR acceptable (if it's not already).

Cheers!