Using a custom yaml loader class

[grst]

It would be nice if I could pass a custom callback function for parsing yaml files. There's already support for the native yaml loader and the OrderedDict yaml loader and there is a feature branch with support for ruamel. Instead of special-casing the different loaders, there could be an argument to the hiyapyco function that takes a callback function/loader class.

My use case is that I would like to enable "unsafe" loading of some !objects, therefore I'd need a custom loader instead of using safe_load.