replace anonymous struct for Candles

[phamdt]

I'm integrating your library into my application and discovered that I can't access an individual candle from the PriceHistory API. I can only access the list of Candles which references an anonymous struct. In my code it would be convenient if I could pass around the individual candle. I can make the PR if this is acceptable to you.

[zricethezav]

@phamdt

I can make the PR if this is acceptable to you.

Go for it 😄