Added a DateInterval type that is just a range between 2 dates with iteration
Added dayofweekinmonth, firstdayofweek, and lastdayofweek generic date functions. Will be useful with the planned recur() function
Shortened the today() function to one-line
Changed how Periods are shown adding their 'units'. Fixes #1
Added DateRange1 type that has an anonymous function as its step. Also adds the recur() function which provides the ability to create irregular date range frequencies, a la runt/Temporal expressions.