Modularize the source code.

This makes greeting generation separate from greeting output, which
makes it easier to unit-test greeting generation.

Change-Id: Ice07f45ed0e9274bd4d4b3cd031cb37fffd34a5e
diff --git a/mod/greeting.hpp b/mod/greeting.hpp
new file mode 100644
index 0000000..c9d1a3d
--- /dev/null
+++ b/mod/greeting.hpp
@@ -0,0 +1,9 @@
+#pragma once
+
+#include <string>
+
+namespace greeting {
+
+std::string make_greeting(std::string const& greetee);
+
+}