Add max_length option to transliteration_clean_filename

Issue Summary

It turns out that Excel 2003 cannot open files with a filename longer that 40 character. I suspect there are more less frequently used with such a "weird" restriction.

(It's a display title turned into a filename here btw)

It'd be nice to have a max_length option that would default to no action, so that it is backwards compatible.

If a number is given the filename would then be just chopped off at that point.