Adding support for x-include-refresh-token-hard-expires-in #56

Hello!

Regarding: https://medium.com/intuitdev/track-your-refresh-tokens-hard-expiry-with-a-new-response-field-7a95ed3ab67f

I have built a wrapper around intuitlib.client.AuthClient that subclasses both refresh and get_bearer_token with adding a new param include_refresh_token_hard_expires_in=False. When set to True is adds the necessary header field. It also adds self.x_refresh_token_hard_expires_in = None to the init.

Would you be open to a pull request to get this into the official version? I see in the blog it's coming soon. Not sure if that means it's already in the works or not.

Thanks.