Add a default value for maxage-days

[ben-rowan]

Fixes #49

Happy to change the default value but this fixes the issue.

[ben-rowan]

Actually I've just noticed the following from the README:

podfox prune will clean up episodes that are too old. If a max age is not set via parameter to command or in the configuration, then no pruning is done.

Would you prefer this is handled by checking if the value is set rather than ensuring it is always set?

[antfred]

I faced the same issue when merging my branch, I think there should be a default value to solve it but I would rather prefer it to be 0 days so that nothing happens unless the user sets a different one explicitly