New method: Guess.gendered_form_of

I added a new method which I needed for my use of this gem. It allows you to do this:

    Guess.gendered_form_of(:they, "Don Draper")
    # => "he"
    Guess.gendered_form_of(:their, "Betty Draper")
    # => "her"
    Guess.gendered_form_of(:theirs, "Mad Men")
    # => "theirs"

So you're welcome to add this to the main gem if you like.