cross-platform size_threshold_ignore

[ArnoStrouwen]

When specifying size_threshold_ignore as my_folder/my_file.md on Windows, I still get errors about the file being too large.
These errors go away if I use my_folder\\my_file.md instead.

This is different from the behavior of the pages keyword of makedocs, which is cross-platform, and it would be nice if the exact same string could be used for both keywords.