Accept '.yaml' extension for the application manifest

`.yaml` is the preferred extension for YAML. See http://www.yaml.org/faq.html:

>Is there an official extension for YAML files?
Please use ".yaml" when possible.

Note that we still default to selecting `manifest.yml` if both exist. Considering a lot of people have editor actions associated with the `.yaml` extension (and thus find it easier to edit a `manifest.yaml`), would you consider merging this patch?