Accept additional type inputs for to & from_param

[bsolomon1124]

Accept date, datetime, float, int, str, or None for both date-related parameters.

The input will be converted to either an ISO-8601 date string or datetime string using utils.stringify_date_param().

A couple possible TODOs (at least to document):

• This assumes the datetime input is UTC (whether it is naive or tz-aware).
• This does not do additional validation of date as str (i.e. validating that month is in 1..12.