Change default loader for YAMLObject classes to FullLoader

[perlpunk]

Fixes #266

The default loader for yaml.load is yaml.FullLoader, so the default
for the YAMLObject class should also be changed.

[kgutwin]

This is a good short-term fix, but you’ll still have some users confused as to why their use of YAMLObject as shown in the documentation doesn’t work with yaml.safe_load (for example). The documentation should also be updated to explain when it would make sense for users to set the yaml_loader and yaml_dumper variables.

[ingydotnet]

@perlpunk @kgutwin Please take a look at #279.

[perlpunk]

Closing, replaced by #279