streak-mode: let <space>, <enter> fall through

- <space>, <enter> are commonly bound to useful context-sensitive functions
- use <esc> or ctrl-c to exit streak-mode without fall-through
streak-mode: ";" should be the first label

- saves a label
- semantically equivalent to ";" behavior
- works for any key mapped to <Plug>SneakNext, if you add it to
  g:sneak#target_labels