Add Spotify intergration colour

Added property `colour` and alias `color` which returns the Spotify
integration colour (#1db954).

Technically Discord uses both (#1cb050 and #1db954) but it appears the
former is an official Spotify colour.

discord/activity.py

@@ -25,6 +25,7 @@
 """
 
 from .enums import ActivityType, try_enum
+from .colour import Colour
 import datetime
 
 __all__ = ('Activity', 'Streaming', 'Game', 'Spotify')
@@ -456,6 +457,20 @@ def type(self):
         """
         return ActivityType.listening
 
+    @property
+    def colour(self):
+        """Returns the Spotify integration colour, as a :class:`Colour`.
+
+        There is an alias for this named :meth:`color`"""
+        return Colour(0x1db954)
+
+    @property
+    def color(self):
+        """Returns the Spotify integration colour, as a :class:`Colour`.
+
+        There is an alias for this named :meth:`colour`"""
+        return self.colour
+
     def to_dict(self):
         return {
             'flags': 48, # SYNC | PLAY