streak-mode operator support

- operators now work for streak-mode motion (eg: dzab{target})
- closes #59
streak-mode: put 's' at the start of target_labels

- gains an extra target label without changing clever-s
- avoid feedkeys() for `;`
streak-mode: allow S/Z/f/F/t/T in target labels

- only disallow s/;/,
- no need to prevent visual-mode mappings
- backwards-clever-s probably won't be missed at the streak _prompt_
handle very long wrapped lines

https://github.com/Lokaltog/vim-easymotion/issues/82
borrow strdisplaywidth() from tomtom