Add support for full RGB colors.

FTXUI supported only the 16 colors palette.
This patch adds support for the 256 palette and the TrueColor(8×8×8)
mode.

This was made by kerdelos@ and fixes issue:
https://github.com/ArthurSonzogni/FTXUI/issues/45

Co-authored-by: Damien D <kerdelos@gmail.com>
Co-authored-by: Arthur Sonzogni <sonzogniarthur@gmail.com>

include/ftxui/screen/terminal.hpp

@@ -0,0 +1,24 @@
+#ifndef FTXUI_CORE_TERMINAL_HPP
+#define FTXUI_CORE_TERMINAL_HPP
+
+namespace ftxui {
+
+class Terminal {
+ public:
+  struct Dimensions {
+    int dimx;
+    int dimy;
+  };
+
+  static bool CanSupportTrueColors();
+
+  static Dimensions Size();
+};
+
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_CORE_TERMINAL_HPP */
+
+// Copyright 2020 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.