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"

man/man1/fzf.1

@@ -219,7 +219,7 @@ e.g. \fBfzf --bind=ctrl-j:accept,ctrl-k:kill-line\fR
 .RE
 
 .RS
-  \fBACTION:               DEFAULT BINDINGS:
+  \fBACTION:               DEFAULT BINDINGS (NOTES):
     \fBabort\fR                 \fIctrl-c  ctrl-g  ctrl-q  esc\fR
     \fBaccept\fR                \fIenter   double-click\fR
     \fBbackward-char\fR         \fIctrl-b  left\fR
@@ -249,6 +249,8 @@ e.g. \fBfzf --bind=ctrl-j:accept,ctrl-k:kill-line\fR
     \fBtoggle\fR
     \fBtoggle-all\fR
     \fBtoggle-down\fR           \fIctrl-i  (tab)\fR
+    \fBtoggle-in\fR             (\fB--reverse\fR ? \fBtoggle-up\fR : \fBtoggle-down\fR)
+    \fBtoggle-out\fR            (\fB--reverse\fR ? \fBtoggle-down\fR : \fBtoggle-up\fR)
     \fBtoggle-sort\fR           (equivalent to \fB--toggle-sort\fR)
     \fBtoggle-up\fR             \fIbtab    (shift-tab)\fR
     \fBunix-line-discard\fR     \fIctrl-u\fR

src/options.go

@@ -566,6 +566,10 @@ func parseKeymap(keymap map[int]actionType, execmap map[int]string, toggleSort b
 			keymap[key] = actToggleDown
 		case "toggle-up":
 			keymap[key] = actToggleUp
+		case "toggle-in":
+			keymap[key] = actToggleIn
+		case "toggle-out":
+			keymap[key] = actToggleOut
 		case "toggle-all":
 			keymap[key] = actToggleAll
 		case "select-all":

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()