Add API to set terminal fallback size (#230)

In case of embedded systems, the terminal size may not
always be detectable (e.g. in case of serial output).
Allow application to set up the default size in case
autodetection fails. On platform such as Emscripten,
there is only "fallback" size.

Signed-off-by: Jarosław Pelczar <jarek@jpelczar.com>

include/ftxui/screen/terminal.hpp

@@ -9,6 +9,7 @@ struct Dimensions {
 
 namespace Terminal {
 Dimensions Size();
+void SetFallbackSize(const Dimensions& fallbackSize);
 
 enum Color {
   Palette1,