Some user galleries [*] require you to be either logged in or
authenticated via OAuth2 to access their deviations.
[*] e.g. https://polinaegorussia.deviantart.com/gallery/
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known issue:
A deviantart 'refresh_token' can only be used once and gets updated
whenever it is used to request a new 'access_token', so storing its
initial value in a config file and reusing it again and again is not
possible.
- add wait-time between HTTP requests similar to exhentai
- add 'wait-min' and 'wait-max' options
- increase retry-count for HTTP requests to 10
- implement user authentication (non-authenticated users can only view
images up to page 25)
- implement 'skip()' functionality (only works up to page 50)
- implement image-retrieval for pages >= 51
- fix issue with multiple tags
