Fix and improve do_truncate

Fix and improve do_truncate

- Ensure that the requested max length be at least as long as the
  requested `end` value (e.g. 3 in the case of '...'). Fixes #539
- Add `leeway` parameter so that strings that just barely miss the
  requested max length cutoff can still be spared from unwanted truncation.
  Default value is 5.
- No longer append a space before appending `end` under any circumstances.
  Adding whitespace before ellipsis punctuation in English is grammatically
  incorrect.