Add a more flexible way to adjust Magic or deal with new/alternate magic numbers.

[rocky]

Right now xdis needs to be update every time a new Python release comes out. This is by design because we do want to have this program be a reference on valid Python's rather than hypothetical ones.

However one proble is that right now we can only handle one magic number per Python version and sometimes for a given Python version there can be more than one. See for example #69.

Propose and implement a way to accomodate these problems.

One possibility is to add a parameter to override the built-in magic to Python mappings. And an option to pydisasm could allow a magic number override.

No doubt there are a number of other possibilityes.