add Channel#delete_messages

[z64]

allows for selective pruning of messages in a channel (instead of just last N messages) to make better use of the bulk_delete endpoint, for example, unless a Member has a certain role:

command(:prune_unless_role, 
        required_permissions: [:manage_messages]) do |event|
  role = event.server.roles.find { |r| r.name == 'moderator' }
  messages = event.channel.history(100).select do |m|
    m.author.role? role
  end
  event.channel.delete_messages(messages)
end

[meew0]

This can be replaced with messages.map!(&:resolve_id), as that is what the resolve_id function is intended for. (Bonus feature: it allows IDs as strings too)