Add the ability for the invite page to be injected with channels

Hey, I will try to get this working, but it would be awesome if I could inject the invite page with channels. This means that there also needs to be a channel section of the invite. 

But the use case would be: I have a "Lets talk on slack": badge on my repo that points to the slack invite url injected with the channel name that my repo uses to talk i.e. documentation of documenter and such. 