Add support for conversion to/from the Jalali calendar

This PR extends the Hijri goodie to add support for conversions to/from the Jalali calendar.  The Jalali calendar is mostly used in Iran these days.  For the conversion, I use the Date::Jalali2 perl module.

Please note that this is my first contribution to ddg, so I hope that I have followed all of the rules etc.  Also, I don't know much Perl, so please review my Perl very carefully and feel free to suggest things that I could do better, and I'd be happy to address them.  Thanks!
No big worries, but I think this name might be a _bit_ too generic for what it actually does.  Does it make sense to call it "CalendarConversion" or something similar?