Add toggle-in and toggle-out for --bind

Related: #452

When `--multi` is set, tab key will bring your cursor down, and
shift-tab up. But since fzf by default draws the screen in bottom-up
fashion, one may feel that the opposite of the behavior is more
desirable and choose to customize the key bindings as follows.

    export FZF_DEFAULT_OPTS="--bind tab:toggle-up,shift-tab:toggle-down"

This configuration, however, becomes no longer straightforward when
`--reverse` is set and fzf switches to top-down layout. To address the
requirement, this commit adds `toggle-in` and `toggle-out` option which
switch direction depending on `--reverse`-ness.

    export FZF_DEFAULT_OPTS="--bind tab:toggle-out,shift-tab:toggle-in"