Add YAML exporter.

This exports the foreman configuration in a YAML file that other tools can read, so the tools don't need to understand how to parse a Procfile.

My concrete use case is that I want to write a tool that tells me what ports are assigned on a server. I plan to run `foreman export yaml` at the same time as `foreman export systemd`, with the exact same parameters, and keep the .yml file around and use it with other tools.

Would you please consider merging this feature?