Support explicit plural forms at client side l10n

Adds support for explicit plural forms like n=form

Example:
message: There {{plural:$1|is one car|are $1 cars|0=are no cars|12=are a dozen
cars}}

with $1=12, this gives: There are a dozen cars

Includes qunit tests.

Bug: 51284
Change-Id: I82b906769ccdd9907d2d8022a689232f496c033e
Merge "Support explicit plural forms at client side l10n"