Retrieve all subscriptions

[nmattia]

This renames the current getSubscriptions to getSubscriptionsByCustomerId and introduces a new getSubscriptions:

-- | Retrieve all active `Subscription`s
getSubscriptions
    :: StripeRequest GetSubscriptions

which retrieves all the subscriptions (for all customers).
Fixes #83

[dmjio]

@nmattia Thanks 🍻 !

[dmjio]

@nmattia your changes are live on 2.3.0, if there's anything else you need, don't hesitate to ask.

[nmattia]

@dmjio thanks a lot for the fast reaction, and for the great library!

[dmjio]

@nmattia thanks for using it 👍

[nmattia]

Ah, I actually came up with some items (since you asked :) )

• Add support for http-client #90
• Add support for stripe pagination #89

Will be happy to pay in 🍻 if you swing by https://zurihac.info/

[dmjio]

I unfortunately won't be at ZuriHac, but I do accept payment in other forms as well 😄