Add method to add your own custom emoji after loading the library

I wanted to add a custom emoji and found out the only way to do so, is to fork the repo and add the image. Judging by other pull requests in this repo other people (for example #42, #36, #34 etc) are looking to do the same. 

Since you no longer accept pulls from custom emoji I worked something up that allows you to add emoji in your code while keeping the gem untouched.

Couple of things:

1. Are you interested in something like this? 
2. I'm not sure about the way I implemented it right now, feels a bit dumb. But it works though :+1: 

Would love to have your feedback and thought on this.