Add message bulk delete handler, make member attribute optional on deleted messages.

[Jack073]

This PR allows you to handle message bulk delete events, as well as adds checks to avoid runtime panics from a nil pointer dereference if a message edit/delete handler isn't set.

It also changes the behaviour of the deleted message handler slightly, previously it guaranteed that the Member attribute of the message was set, however this meant for example if someone left a server and a mod or staff member deleted one of their messages, it wouldn't be logged passed to the DeletedCallback. The Author attribute will always be set, and it will try to set the Member attribute, but it will be nil if there was an error getting the member. The edited message handler remains unaffected besides from adding the nil handler check mentioned above.

This new behaviour partially comes from the new bulk delete handler where (for consistency and ease of use) it fetched the member and set the member on the message. There was 3 main options to handle members that couldn't be fetched:

1. returning from the function here as the single message delete handler did (though you would miss out on any bulk deletes if even one of the message authors had left the server)
2. continuing to the next message, ignoring this message, but this would also cause missing messages is someone left the server
3. make the Member attribute optional, but try to add it, adding the Author attribute anyway.

This does however add another method to the MessageCacheStorageAdapter interface - BulkGetAndDelete(ChannelID disgord.Snowflake, MessageIDs []disgord.Snowflake) []*disgord.Message, so this will be a breaking change if you use a custom MessageCacheStorageAdapter, however it does enable easy logging / handling of message bulk deletes.

[IAmJSD]

LGTM