When streamers run ads, they'd like to know how long the PreRolls are disabled for.

Please extend https://dev.twitch.tv/docs/api/reference#start-commercial to return preroll data

For example

{
"data": [{
"length" : 60,
"message" : "",
"retry_after" : 480
}]
}

becomes

{
"data": [{
"length" : 60,
"message" : "",
"retryafter" : 480,
"prerollends": unixTimestamp
}]
}