fzf-marks complains if FZF_MARKS_FILE points at a readonly file

[peterhoeg]

If for whatever reason that file is readonly, fzf-marks will throw a warning about not being able to do inplace editing. It still works as normal though.

[urbainvaes]

This is because the directories are sorted from the most recently to the least recently visited, in order to make it easier to jump to recently used marks. We could add an option to enable/disable this behaviour explicitly, which could default to 'disable' if the file is readonly and 'enable' otherwise. What do you think?

[peterhoeg]

That's definitely an option.

Ideally the list of directories is separate from the ordering so that the it can be kept readonly while the dynamic ordering data is stored in a rw location.