This command should reply to every message that starts with Nightbot.
!commands add nightbot $(user) $(urlfetch https://api.rtainc.co/twitch/random?format=&choices=yes,no)
A few notes, it will only respond if nightbot is the first word, you can make it respond to @nightbot as well:
!commands add -a=nightbot @nightbot $(query)
The command above will link with an alias to the command I showed first, so you need to have the first one added. Same story, it only works if the message starts with @nightbot.
This command uses an urlfetch url from this topic, which picks a random value from the comma seperated list: Useful Custom APIs