Add `remaining` method to limiter

[julianrubisch]

Working with Billing::Limiter, I thought it would be handy to have a convenience method spitting out the remaining count of allowed models.

Currently I'm solving this like so:

limiter.limits_for(:have, MyModel).map { _1["count"] }.max - team.my_model.count

WDYT?

[jagthedrummer]

I think that would be great!