Create new ContextStore interface

[derekperkins]

Since most "Stores" will require an external request, the interface should implement context.Context so that those requests can respect timeouts and cancellations. I'd expect it to look something like this.

// ContextStore is the common interface for limiter stores.
type ContextStore interface {
	Get(ctx context.Context, key string, rate Rate) (Context, error)
	Peek(ctx context.Context, key string, rate Rate) (Context, error)
}

Honestly, it would be great if the whole package went v2 and implemented context as the first parameter everywhere, including the middleware functions.

[novln]

Hello Derek,

A v2 is on it's way, we will add context as first parameter for the Store interface.

Thank you for your feedback :)