Included .yml as supported extension. 

Hi. Thanks for creating Viper, it's been very useful to me. I come from Ruby and .yaml is really never used there, .yml is the common extension. I verified online and it's valid known extension for Yaml files so I added it. I realized this when trying to use `config.yml` in a project and getting an error on my code. Does adding this makes sense? Pretty silly addition, but it did confuse me for a while to realize the error using my app on a server, so I think it's worth adding. Cheers.