Prompt for and send a message to a channel.

def send_message(slack):
    """ Prompt for and send a message to a channel. """
    channel = input('Which channel would you like to message? ')
    message = input('What should the message be? ')
    channel_id = channel_to_id(slack, channel)

    print(f"Sending message to #{channel} (id: {channel_id})!")
    slack.rtm_send_message(channel_id, message)