Add support for additional filters like active

[rnmp]

Heyo.

I just started using this gem and I noticed that things like Stripe::Plan.list(active: true) will get ignored and return the full list instead. I checked out the code and noticed there's no logic around filters like active in list.rb.

Other than that, thanks for making this gem, it's making my life easier, especially since this is the first time I'm building for Stripe.

EDIT: Made the change myself since you guys are in search for contributors :P