add a function to get the daily double indices

[tbradley1013]

This function is pretty straightforward and is pretty much just copied from your whatr_scores function. But instead of getting all the extra information, it just returns the indices of the daily doubles.

This is useful if you don't particularly care about the scores of the people but want to quickly get which questions were daily doubles

[k5cents]

Looks good. I'll merge this and tweak the docs a bit.