more-precise mapcheck()
conservative mapping behavior

- provide visual-mode <Plug> mappings: <Plug>VSneakForward, <Plug>VSneakBackward
- don't map s/Z in visual mode if they are already mapped
- allow disabling of the 'z' textobject:
    let g:sneak#options.textobject_z = 0
fix typo
fix readme

thanks NoahTheDuke