add fitbit goal: minutes in heart rate "cardio zone"
Spending a certain amount of time per week with your heart rate around 130 beats a minute is a very common way to discuss low-intensity fitness/weight-loss programs. Tracking this is one of the major reasons I decided to get a fitbit.
Fitbit calculates and exposes this data: https://dev.fitbit.com/docs/heart-rate/
As a web developer I kinda wish there was a `beeminder-contrib` I could open a pull request on...