Get access to rate limit headers

[pcothenet]

(follow-up from #23).

The Clearbit API returns rate limit information inside the HTTP Headers.
AFAIK: this SDK does not surface the header in the response. Is there a way to access those headers (e.g. to figure out the correct backoff) from node-clearbit or do we need to implement our own version to call the API directly and access the headers?

[tommedema]

This is much needed