loader: require .json extension and allow .yaml extension

[rmilecki]

loader: allow YAML templates with .yaml extension

Oficially suggested extension for YAML files is .yaml. Accept it.

loader: require .json extension for JSON files

Trying to parse every file other than *.yml as JSON file isn't the best
idea. It wastes time on trying to parse non-template files. It produces
warnings like "json Loader Failed to load (...)template" for files that
were never meant to be templates. It's a bad pracice in general.

Check for file extension before using json.loads().

[bosd]

Thanks for this PR.
I agree the loader could use some work. It's on my wish list :)
As the recent refactoring broke the possibility to stream load templates.

I think adding more extension filters will block the possibility to streamload templates even further.