[RFC] Add option for streak mode with `<Plug>Sneak_f` etc

This allows use (and force) streak mode with `f` / `t` etc:

    let g:sneak#streak=1
    let g:sneak#streak_with_f=2
    let g:sneak#streak_with_t=2

This PR misses documentation and tests, because it's meant to discuss this first.

I could also imagine to make `g:sneak#streak=2` trigger this.