[deviantart] Request: given a refresh-token, let gallery-dl download Watchers only images

[Miroslav-Cetojevic]

In reference to #1463 I'd like to request the ability to download Watchers only images. In terms of gallery-dl.conf I'm thinking of something like this:

extractor.deviantart.watch

Type: bool

Default: true

Comment: For Watchers only! Lets user add artist to the watchlist before downloading their image. Only works if user has set up a refresh-token.

extractor.deviantart.unwatch

Type: bool

Default: true

Comment: For Watchers only! Lets user remove artist from the watchlist immediately after their image has been downloaded. Only works if user has set up a refresh-token. Also, extractor.deviantart.watch must be set to true.

Setting both options to true by default means the user doesn't have to think about the watchlist unless they WANT to follow any artists that require viewers to put them on their watchlist.