I have never used Twitch but heres a code that works on a IRC network via mIRC

Place into remotes and do

!topic message.... E.g. !topic This is a test channel
!showtopic - Will show the topic for that channel

Must have op level to use the command.

Will work for as many channels you want but it you want to make it to work in on channel where it says on *:TEXT:*:#: { change it to on *:TEXT:*:#CHANNEL NAME: {


Code:

on *:TEXT:*:#: { 
  if ($strip($1) == !topic) {
    if ($nick !isop $chan) { .notice $nick Sorry but you must be opped to use this command! | halt }
    if ($2 == $null) { .notice $nick Incorrect Usage! To use this command do !topic topic message. Example !topic Welcome to #Test Channel |  halt }
    else {
      .topic $chan $2-
      .writeini -n topic.ini $chan Topic $2-
    }
  }
  if ($strip($1) == !showtopic) {
    if ($nick !isop $chan) { .notice $nick Sorry but you must be opped to use this command! | halt }
    if ($readini(topic.ini, $chan, Topic) !== $null)  .notice $nick The topic for $chan is $readini(topic.ini,$chan, Topic) 

  }

Regards

JayStew