API: support "channel" field in incoming JSON payload

Ryver could instantly support a wide range of existing Slack integrations if it replicated the "channel" field a la the Slack API: https://api.slack.com/incoming-webhooks#customizations_for_custom_integrations (scroll down a few lines to the "Channel override" section).

In short, if "channel" is specified in the JSON for an incoming message, the room specified in the webhook URL would be overridden and the message would be directed to the appropriate room.

There are many existing Slack integrations that direct messages in this way, so by adding this field to the Ryver API, Ryver could instantly support all of these Slack integrations with no special setup required from users.