Add support for config files with the "yml" extension ("yaml" still works)
Only check for a ".yaml" file when the appropriate ".yml" file is missing
Merge pull request #11 from thenickperson/support-yml-extension

Add support for config files with the `yml` extension
 - As `yml` appears to be the defacto standard this is the default now
 - Will look for `yaml` files of `yml` doesn't exist