Selective FactoryGirl.lint

I'm trying to scratch my own itch with this one, maybe i'm not alone with this issue. We're using virtus models heavily and we're using factory girl for its `attributes_for` feature and building virtus models with it. Linting breaks with this setup, so i'm proposing syntax like this for filtering linted factories:

    config.before(:suite) do
      FactoryGirl.lint except: [:post]
    end

It will lint all factories for all models except `Post`. I think it'll be useful with other kinds of non-so-standard installations too. If you'll be willing to merge this PR, i'll add documentation to this feature and description of possible use cases. Also, i can add `only` key to compliment `except`.