Using dateutil.parser prevents some helpful date offsets

[adamlincoln]

While generating a date field with the date_between function block, I can use a min/max offset value of days, like the documentation says:

SomeFieldName:
  date_between:
    min: -120d
    max: -90d

but I can't use month as the unit for this offset, even though faker's date_time provider supports it. If I try:

SomeFieldName:
  date_between:
    min: -4M
    max: -3M

Then I silently get the wrong result: for min, a datetime with today's date, and 00:04:00 as the time; for max, a datetime with today's date, and 00:03:00 as the time.

It looks like this is because snowfakery sends the value through dateutil.parser.parse() first (in template_funcs:parse_date()). For a days-back value like -120d, dateutil.parser.ParserError is raised so -120d gets passed to faker and everything works. For a months-back value like -3M, dateutil.parser doesn't raise ParserError and returns an unexpected datetime.

Expected behavior: snowfakery's date values support all of faker's formats. Note that snowfakery's documentation says values like -4m are supported for months, but this has the same output as described for -4M above, and if faker's formats were followed, -4m would be four minutes ago.