Add nano-style key menu option

Use the `keymenu` option (default `off`) to enable. ToggleKeyMenu is
also bound to `Alt-g` and this info is now displayed in the status line.

Closes #829

cmd/micro/tab.go

@@ -36,6 +36,9 @@ func NewTabFromView(v *View) *Tab {
 	if globalSettings["infobar"].(bool) {
 		t.tree.height--
 	}
+	if globalSettings["keymenu"].(bool) {
+		t.tree.height -= 2
+	}
 
 	t.Resize()
 
@@ -62,6 +65,9 @@ func (t *Tab) Resize() {
 	if globalSettings["infobar"].(bool) {
 		t.tree.height--
 	}
+	if globalSettings["keymenu"].(bool) {
+		t.tree.height -= 2
+	}
 
 	t.tree.ResizeSplits()