Implement ButtonOption::Border() (#472)

It was missing. See:
https://github.com/ArthurSonzogni/FTXUI/issues/471

CHANGELOG.md

@@ -22,6 +22,7 @@ current (development)
     - various values (value, min, max, increment).
 - Improvement: The `Menu` keeps the focus when an entry is selected with the
   mouse.
+- Bug: Add implementation of `ButtonOption::Border()`. It was missing.
 
 ### Screen
 - Feature: add `Box::Union(a,b) -> Box`

src/ftxui/component/component_options.cpp

@@ -144,6 +144,23 @@ ButtonOption ButtonOption::Simple() {
   return option;
 }
 
+/// @brief Create a ButtonOption. The button is shown using a border, inverted
+/// when focused. This is the current default.
+ButtonOption ButtonOption::Border() {
+  ButtonOption option;
+  option.transform = [](const EntryState& s) {
+    auto element = text(s.label) | border;
+    if (s.active) {
+      element |= bold;
+    }
+    if (s.focused) {
+      element |= inverted;
+    }
+    return element;
+  };
+  return option;
+}
+
 /// @brief Create a ButtonOption, using animated colors.
 // static
 ButtonOption ButtonOption::Animated() {