www/rubygem-tumblr_client: create port

This is the official Ruby wrapper for the Tumblr v2 API.
It supports all endpoints currently available on the Tumblr API.

WWW: https://github.com/tumblr/tumblr_client

PR: 195758
Differential Revision: https://reviews.freebsd.org/D1278
Submitted by: Michael Moll <kvedulv@kvedulv.de>
Approved by: kswills (mentor)