[deviantart] add support for OAuth2 authentication

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/

--------------

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.

docs/configuration.rst

@@ -271,7 +271,7 @@ Description Controls the behavior when downloading a file whose filename
 
 
 extractor.*.sleep
-----------------
+-----------------
 =========== =====
 Type        ``float``
 Default     ``0``
@@ -370,6 +370,19 @@ Description Request full-sized original images if available.
 =========== =====
 
 
+extractor.deviantart.refresh-token
+----------------------------------
+=========== =====
+Type        ``string``
+Default     ``null``
+Description The ``refresh_token`` value you get from linking your
+            DeviantArt account to *gallery-dl*.
+
+            Using a ``refresh_token`` allows you to access private or otherwise
+            not publicly available deviations.
+=========== =====
+
+
 extractor.exhentai.original
 ---------------------------
 =========== =====
@@ -576,10 +589,10 @@ extractor.reddit.refresh-token
 =========== =====
 Type        ``string``
 Default     ``null``
-Description The ``refresh_token`` value you get from linking your Reddit account
-            to *gallery-dl*.
+Description The ``refresh_token`` value you get from linking your
+            Reddit account to *gallery-dl*.
 
-            Using the ``refresh_token`` allows you to access private or otherwise
+            Using a ``refresh_token`` allows you to access private or otherwise
             not publicly available subreddits, given that your account is
             authorized to do so,
             but requests to the reddit API are going to be rate limited