Made Clara a “Cliche” header, which will be usable independently of Catch.

- This is just a first step. It still has a dependency on catch_text.h, which also needs to be made a Cliche header.
- These then need their own homes on GitHub.

include/internal/catch_commandline.hpp

@@ -10,7 +10,7 @@
 
 #include "catch_config.hpp"
 #include "catch_common.h"
-#include "clara.h"
+#include "catch_clara.h"
 
 #include <fstream>