BUG #14850: Implement optinal additinal parameter for 'justify' date/time function

The following bug has been logged on the website:

Bug reference: 14850
Logged by: Eugen Konkov
Email address: kes-kes(at)yandex(dot)ru
PostgreSQL version: 10.0
Operating system: Linux mint 18: Linux work 4.4.0-57-generic #78-Ubu
Description:

Hi. I try to do next math:

select extract( month from justify_days( timestamp '2016-01-31' +interval '1
month' -timestamp '2016-01-31') );
date_part
-----------
0
(1 row)

I expect `1` but get `0`. But here everything is right:

>Adjust interval so 30-day time periods are represented as months

https://www.postgresql.org/docs/9.6/static/functions-datetime.html

But with ability to setup justify date the math will be more sharp.

Please implement next feature:

select extract( month from justify_days( timestamp '2016-01-31' +interval '1
month' -timestamp '2016-01-31'), timestamp '2016-01-31' );
date_part
-----------
1
(1 row)

This is useful when I try to calculate how much month are left between
service start and end dates.

Thank you.