Support full-width characters (CJK)

+ 3 tests.

The goal is to fix:
https://github.com/ArthurSonzogni/FTXUI/issues/2#issuecomment-504871456

include/ftxui/screen/string.hpp

@@ -1,3 +1,6 @@
+#ifndef FTXUI_SCREEN_STRING_HPP
+#define FTXUI_SCREEN_STRING_HPP
+
 #include <string>
 
 std::string to_string(const std::wstring& s);
@@ -7,3 +10,10 @@ template<typename T>
 std::wstring to_wstring(T s) {
   return to_wstring(std::to_string(s));
 }
+
+int wchar_width(wchar_t);
+int wchar_width_cjk(wchar_t);
+int wstring_width(const std::wstring&);
+int wstring_width_cjk(const std::wstring&);
+
+#endif /* end of include guard: FTXUI_SCREEN_STRING_HPP */