Allow user to specify encoding

[einstein95]

This is my attempt at providing an updated version of #124. Comparing to that PR, I have:

• added the encoding param to the documentation for the internal functions
• defaulted the encoding param to an empty string so that using that variable within the function won't give any linter errors
• added encoding type checking to get_file_from_iso in line with that section of the code
• removed useless None default when using .get as the default is None
• fix code in walk which ignored the default encoding when calling self.list_children

Test results as output by Github Actions:

====================== 1743 passed, 17 skipped in 10.99s =======================

[sev-]

It would be really nice to have this merged. We are using your library in the ScummVM project (https://scummvm.org), and this is one of the problem we faced with: the Hybrid Mac/Windows CDs with Japanese filenames have the encoding lost.