datetime param to be optional. current time by default

Is there any reason for HRForecast api being required 'datetime' instead of optional?

If 'datetime' would be a optional parameter, HRForecast could have growthforecast
compatible interface, and easily cooperate fluent-plugin-growthforecast and so on.

Is there no big reason, I hope 'datetime' to be a optional parameter.

If this patch is not match HRForecast's philosophy, please reject it.