Add join_key arg to source plugin get_artist method

Add an optional argument to MetadataSourcePlugin.get_artist method that enables
making use of a field containing a keyword supposed to be used to combine
artists together into a single string like "Feat.", "And", "Vs." and so on.

beets/plugins.py

@@ -655,7 +655,7 @@ class MetadataSourcePlugin(metaclass=abc.ABCMeta):
         raise NotImplementedError
 
     @staticmethod
-    def get_artist(artists, id_key='id', name_key='name'):
+    def get_artist(artists, id_key='id', name_key='name', join_key=None):
         """Returns an artist string (all artists) and an artist_id (the main
         artist) for a list of artist object dicts.
 
@@ -663,6 +663,8 @@ class MetadataSourcePlugin(metaclass=abc.ABCMeta):
         and 'the') to the front and strips trailing disambiguation numbers. It
         returns a tuple containing the comma-separated string of all
         normalized artists and the ``id`` of the main/first artist.
+        Alternatively a keyword can be used to combine artists together into a
+        single string by passing the join_key argument.
 
         :param artists: Iterable of artist dicts or lists returned by API.
         :type artists: list[dict] or list[list]
@@ -673,11 +675,17 @@ class MetadataSourcePlugin(metaclass=abc.ABCMeta):
             to concatenate for the artist string (containing all artists).
             Defaults to 'name'.
         :type name_key: str or int
+        :param join_key: Key or index corresponding to a field containing a
+            keyword to use for combining artists into a single string, for
+            example "Feat.", "Vs.", "And" or similar. The default is None
+            which keeps the default behaviour (comma-separated).
+        :type join_key: str or int
         :return: Normalized artist string.
         :rtype: str
         """
         artist_id = None
         artist_names = []
+        joined = False
         for artist in artists:
             if not artist_id:
                 artist_id = artist[id_key]
@@ -686,8 +694,17 @@ class MetadataSourcePlugin(metaclass=abc.ABCMeta):
             name = re.sub(r' \(\d+\)$', '', name)
             # Move articles to the front.
             name = re.sub(r'^(.*?), (a|an|the)$', r'\2 \1', name, flags=re.I)
+            # Use a join keyword if requested and available.
+            if join_key and artist.get(join_key, None):
+                name += " " + artist[join_key]
+                joined = True
             artist_names.append(name)
-        artist = ', '.join(artist_names).replace(' ,', ',') or None
+        # Concat using spaces only if join_key was passed and the join_key was
+        # ever found. Otherwise join comma-separated.
+        if join_key and joined is True:
+            artist = ' '.join(artist_names) or None
+        else:
+            artist = ', '.join(artist_names).replace(' ,', ',') or None
         return artist, artist_id
 
     def _get_id(self, url_type, id_):