provide sneak#cancel()

ref #106
fix #106: <esc> should cancel sneak

- press <esc> to clear highlighting and cause sneak#is_sneaking() to
  return false (0).
- <esc> mapping is temporary and will be removed immediately after,
  so terminal Vim will not be confused.
- sneak will not create the temporary <esc> mapping if you already
  mapped something else to <esc>.