Decide whether users should be able to use a customized format loader map for high-level functions

[xuhdev]

If yes, we need to implement this.

After separating high level function out, we may consider having a high-level function that changes pydax.format_loader_map, which initializes from _default_format_loader_map. pydax.format_loader_map will be used by high-level functions only. The boundaries on how much customization high-level functions should have is still to be discussed.