Add ability to have response type be in_channel so responses can be shared

While reviewing https://github.com/IEvangelist/IEvangelist.Slack.SlashCommands/blob/master/IEvangelist.Slack.SlashCommands/Controllers/JokesController.cs I noticed he was able to provide a response_type to be "in_channel" or "ephemeral" which allows the response to be shared with the Channel and not just visible to the person who sent it.