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.
This is a massive breaking change.
* All references to "game" have been renamed to "activity"
* Activity objects contain a majority of the rich presence information
* Game and Streaming are subtypes for memory optimisation purposes for
the more common cases.
* Introduce a more specialised read-only type, Spotify, for the
official Spotify integration to make it easier to use.
