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"

src/terminal.go

@@ -125,6 +125,8 @@ const (
 	actToggleAll
 	actToggleDown
 	actToggleUp
+	actToggleIn
+	actToggleOut
 	actDown
 	actUp
 	actPageUp
@@ -949,6 +951,16 @@ func (t *Terminal) Loop() {
 					}
 					req(reqList, reqInfo)
 				}
+			case actToggleIn:
+				if t.reverse {
+					return doAction(actToggleUp, mapkey)
+				}
+				return doAction(actToggleDown, mapkey)
+			case actToggleOut:
+				if t.reverse {
+					return doAction(actToggleDown, mapkey)
+				}
+				return doAction(actToggleUp, mapkey)
 			case actToggleDown:
 				if t.multi && t.merger.Length() > 0 {
 					toggle()