Feature: Add support for interval parameter to OHLC function

[primeroz]

Hi, at the moment the OHLC function https://github.com/beldur/kraken-go-api-client/blob/master/krakenapi.go#L169 does not allow passing an interval to it, so is only possible to use the default value of "1" which limits the returned data quite a bit.

I did open #59 but then closed it because it would force everyone to specify an interval even when they actually want to use the default

I am not sure what the right way to handle this use case is

• Using an args map[string]string make no sense, since the only other param supported is interval itself
• Create a separate OHLCWithInterval function to handle all the current code in OHLC and have that one just call the new one with itnerval hardcoded to 1 - something like #60
• Just add the new parameter as in my previous PR

I would be curious to know what you think

thanks!