Add Spotify intergration colour
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.

Harmon758/discord.py,Harmon758/discord.py,rapptz/discord.py,Rapptz/discord.py,khazhyk/discord.py,imayhaveborkedit/discord.py