Feature: hscroll_indicator (#753)

This is the symetrical of `vscroll_indicator`.

Requested by @ibrahimnasson.

Fixed:https://github.com/ArthurSonzogni/FTXUI/issues/752

src/ftxui/dom/color.cpp

@@ -12,7 +12,7 @@
 
 namespace ftxui {
 
-  namespace {
+namespace {
 class BgColor : public NodeDecorator {
  public:
   BgColor(Element child, Color color)