Adding suffix to disambiguate URLs can push total slug length outside the limit

[fauxparse]

As demonstrated in test_handling_duplicate_urls_with_limits, where the :limit is given as 13, but the resulting string is 15 characters long.

I'm happy to work on a fix to address this.

[rsl]

hey there. sorry about delay. if you've got an idea for a fix for this totally welcome. merged in your other change, btw. thanks.