query builder for changes feed in Channels page

[jchris]

To aid developers who want to create async channel workers, we should offer code snippets in the Channels page for how to trigger replication of the current set of channels, and how to query a changes feed with the current set of channels.

It's not easy because it looks like this:

{
  source: "http://sync.example.com/mydb",
  target: "localdb",
  filter: "sync_gateway/bychannel",
  query_params: {
    channels: "comma,seperated,list,of,channel,names"
  }
}

Or like this:

curl http://localhost:4985/chat8/_changes?filter=sync_gateway/bychannel&channels=9hb2i58hjf488,profiles,da40ajgc14h4