Support for custom status

[GeopJr]

Docs: https://github.com/discordapp/discord-api-docs/blob/master/docs/topics/Gateway.md

• Made some emoji struct's fields optional (https://discordapp.com/developers/docs/resources/emoji)
• Added Array of GamePlaying to Presence (because you can now have custom status + something else)
• Added GamePlaying#state and GamePlaying#emoji for reading the custom status

Logs (Presence#activities):

• Custom status with just emoji

[Discord::GamePlaying(@name="Custom Status", @type=4, @url=nil, @state=nil, @emoji=Discord::Emoji(@id=nil, @name="👌", @roles=nil, @require_colons=nil, @managed=nil, @animated=nil))]

• Custom status with just text

[Discord::GamePlaying(@name="Custom Status", @type=4, @url=nil, @state="ok, boomer", @emoji=nil)]

• Custom status + Spotify

[Discord::GamePlaying(@name="Spotify", @type=2, @url=nil, @state="Toby Fox", @emoji=nil), Discord::GamePlaying(@name="Custom Status", @type=4, @url=nil, @state=nil, @emoji=Discord::Emoji(@id=Discord::Snowflake(@value=631306021805228052), @name="hollowblob", @roles=nil, @require_colons=nil, @managed=nil, @animated=false))]

I guess state also has song author