implement getdate as in Python port

[wilsonfreitas]

See https://github.com/mojombo/chronic for inspiration.

[wilsonfreitas]

This is useful for things like

getdate("last bizday", "Brazil/ANBIMA")

[wilsonfreitas]

Day reference can be used with the following specs:

• next <day>: day, bizday, weekday
• last <day>: day, bizday, weekday

with an argument to include the reference date or not.

[wilsonfreitas]

For a ref by_day we can use expr like

• next <day>: day, bizday, weekday
• first <day>: day, bizday, weekday
• 1st/2nd/3rd/nth <day>: day, bizday, weekday

for forward dates, and

• last <day>: day, bizday, weekday

for backward dates.

I need an expression to go further backward. Maybe something like

• last nth <day>: day, bizday, weekday
• nth <day> before/ago: day, bizday, weekday