Granular control options for included holidays

[BryanBaird]

This is much more of a stretch feature, but there were two different aspects that came up in recent analyses for trying to investigate model anomalies.

1. The ability to add in extra holidays for an analysis -- which I believe has started implementation but might not be properly functional yet
2. The ability to exclude certain holidays -- either in general or in a specific year -- to run counterfactual analyses

(2) in particular is less important if other model performance and quantification features are more easily available, but it may still be useful in some cases where certain holidays have unexpected strange patterns associated in given years.

This approach could potentially also allow custom flagging of known historical anomalies that might distort forecasting, but the approach there might instead be to encourage additional contributions to the actual implementation of known anomalies as holidays here:

mozaic-forecasting/mozaic/holidays.py

Line 37 in 2becad6

class MozillaHolidays(holidays.HolidayBase):