Save the API rate limit

[Amatewasu]

Hi! Thank you a lot for your library.

It would be interesting to save the API rate limit and expose it.

Idea for the implementation:

• node-strava-v3, rateLimiting.js
• node-strava-v3, httpClient.js

What do you think of the feature? What do you think of the node-strava-v3's interface?

I guess you prefer if I submit a PR?