DOCTEST_CONFIG_USE_STD_HEADERS in MSVC++ (#30)

* replace <sstream> with <ostream>

* Use provided by doctest macro instead of explicit include (#1)

* dEfine  `DOCTEST_CONFIG_USE_STD_HEADERS` globally

test/source/greeter.cpp

@@ -1,11 +1,6 @@
 #include <doctest/doctest.h>
 #include <greeter.h>
 
-#if defined(_WIN32) || defined(WIN32)
-// apparently this is required to compile in MSVC++
-#  include <sstream>
-#endif
-
 TEST_CASE("Greeter") {
   using namespace greeter;