Remove Plurality enum

[MehdiK]

Plurality enum is only used with Singularize and Pluralize methods and as discussed in #198 is a bit confusing and redundant. We could replace it with a boolean param (e.g. inputIsSingular and inputIsPlural) to clean up the API and remove the unnecessary enum.

[MehdiK]

This is covered in #306. We will remove all the obsolete bits on V2.