Add a way to specify a default for magic_file.

[maringuu]

My use case it the following:
We have some additional magic that we want to always use in our project.
So one idea would be to create an instance of Magic with the appropriate magic_file.
But now when we want to specify mime=True or follow_symlinks=True we need to create a new instance of Magic.
I guess one could say that all other arguments than magic_file are of a more short-lived nature.

This leads to some boilerplate which I think belongs in python-magic instead.

My proposal would be to add some way to set a default value for magic_file.
While I know that this is the way libmagic is designed I think we can to better in a wrapper library.

Related:
#304
fkie-cad/FACT_core#1140

[ahupp]

See my comment on your PR, but I'd support adding an environment variable that sets the default magic file.