Add extraction support for translatePlural filter

[pswai]

trafficstars

This adds support to extract strings from translatePlural filter.

This is together with rubenv/angular-gettext#122

[rubenv]

Really good stuff. If we can make the {{$count}} placeholder work, I'd love to see it used here as well. Apart from that no real remarks.

We still need documentation, but we can add that later on.

[Alir3z4]

:+1: