Support languages with more than 2 plural forms

I’m translating grocy to russian ATM. There are string that can’t be translated into russian cleanly.

Example:

1 Pack
2,3,4... Packs

can’t be translated using only two plural forms as in english, as russian has 3 different plural forms. It should be translated like this:

1,21,31,41,n1 Упаковка
2,3,4,22,23,24,32,33,34,n2,n3,n4 Упаковки
5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,25,26,27,28,29,n5,n6,n7,n8,n9 Упаковок

Otherwise it looks awful:

1 Упаковка
1 Упаков(ки/ок)

Please use ngettext from gettext or other translation frameworks that uses this function.

[ASMfreaK]

Ping me, when there’ll be a version on Transifex, I’ll try to help with russian and other european languages.

[berrnd]

All strings with more than 1 placeholder now have named placeholders.