ignore non-yaml configuration files

[bradrydzewski]

I recommend returning a 204 no-content and ignore repositories if the configuration file (config_path) does not match .yml or .yaml file extension. This will allow the plugin to play nice with native jsonnet and starlark.

(PS sorry for not being more proactive with a pull request, but the Drone backlog is keeping me too busy right now)

[microadam]

Thanks for the suggestion @bradrydzewski! I will add that in when I have some time