Sanitize exported file names to avoid file system errors

[Nightriff]

Including a slash in the object name tries to write a file with a slash in the name, which is an invalid file name and causes the exporter to look for a folder that doesn't exist (as reported on this thread #57 (comment))

There are probably other cases of object names that would also conflict with the file system.

The file name should be sanitized of any special characters that could cause problems with the file system.