TumblrPublishAgent

First attempt at a publish agent for Tumblr. Based very closely off of the TwitterPublishAgent. I'd like some feedback on the structure for inputs as well as the code structure before I start working on additional Tumblr-related agents.
It's totally fine to make a Concern, but if you're only going to have one type of Tumblr Agent, it's not really necessary.  You could include all of this in your `TumblrPublishAgent`.