Add golden hour

[jareyes]

According to http://golden-hour.com/, the Golden Hour is "that period when the sun lies between the horizon and 6 degrees above".

[ghost]

How do you propose to use this? Golden hour start/end in the API?

(I ask since the hourly API already includes solar elevation, which can easily be used to compute this on hourly data points; so I suppose the use-case is events?)

[jareyes]

I was thinking of adding start/end times to events.