Add Channel#topic.
Add Channel#topic.

VxJasonxV/discordrb,VxJasonxV/discordrb,meew0/discordrb,Roughsketch/discordrb,megumisonoda/discordrb,brpeterman/discordrb,brpeterman/discordrb,megumisonoda/discordrb,sarsntony/discordrb,sarsntony/discordrb,Roughsketch/discordrb,meew0/discordrb