Support higher resolution forecast dates

[huddlej]

The current interface only allows users to specify forecasts N months into the future. In practice, we want to produce forecasts at higher resolution like daily or weekly. We can support this functionality by adding an alternate interface for users to specify forecast intervals with ISO duration strings (e.g., 2W or 14D as in pandas or augur) and/or a file with a list of specific dates to forecast. This latter interface would require users to build date lists prior to forecasting which adds an extra barrier to forecasting, but it also provides the most flexible way to specify forecast dates.

If we choose the ISO duration interface, we should use the isodate package.