Add option for indefinite article

This is mainly for those languages which don't have a concept of a plural. To paraphrase a member of the translation team:

> 'song' and 'songs' have the same translation, but 'a song' has a different translation to '650 songs'

As such, we needed a concept of the indefinite article.

there is now a indefiniteSuffix, which will be added to the relevant form, in a language with "standard" pluralisation, keys are:
* key
* key_indefinite
* key_plural
* key_plural_indefnite

For languages that don't pluralise, count != 1 will always return 'key'.