Add annotation code.

Need to figure out how we are getting the annotation data, and how we want to present it to users for use. Is it an additional field where they enter their own endpoint to query, and that endpoint needs to know to respond with unix time and a message?

@juliusv @grobie 

requested in #115/#187
Open question on how to handle this. Failure would be an issue with slog, and the user is unable to fix it. Is it worth it to show them an error message they can't do anything about?