support datetime parameter

Continued from https://github.com/kazeburo/GrowthForecast/pull/64. 

@studio3104 said as we should support the same parameter with HRForecast
https://github.com/kazeburo/HRForecast/blob/master/views/docs.tx#L71-L85

So, I added `datetime` parameter for the API newly. Either of `timestamp` or `datetime` parameter should be used (`timestamp` overcomes).