Add Rot13 in Elixir

[ricardomaps]

To request a new code snippet, please fill out the following:

Project name: Rot13
Project link: https://sampleprograms.io/projects/rot13
Language: Elixir

If this code snippet will also be the first in a new language, please fill out the following:

Official Language Style Guide:
Official Language Website:
Official Language Docker Image: https://hub.docker.com/r/repository/image

The project link must be a link to a project on the Sample Programs Website project list.
If you would like to add a new project to the Sample Programs Website,
please first make a pull request to the Sample Programs Website.