allows src file to be yml, adds tests for this

This allows the src file to be in YAML format. The file extension is used to determine the type. `.yml` and `.yaml` get read by `grunt.file.readYAML`, every other file gets read by `grunt.file.readJSON`, this is not changed.

Tests for this are added using the same fixture data but in YAML format and tested against the same expected output.

If you would like to merge this, I will update the documentation, just let me know.