Moved matcher-based capture macros into their own file

- this file excluded from the CATCH_CONFIG_DISABLE_MATCHERS path.
- matchers are always compiled in to the impl file
- _THROWS_WITH macros are still available with matchers disabled - but only the ones that take a string
- tests that use matchers have #ifdefs, so the whole SelfTest project can compile with matchers disable.

include/internal/catch_matchers_string.cpp

@@ -6,8 +6,6 @@
  * file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 
-#if !defined(CATCH_CONFIG_DISABLE_MATCHERS)
-
 #include "catch_matchers_string.h"
 #include "catch_string_manip.h"
 
@@ -94,5 +92,3 @@ namespace Matchers {
 
 } // namespace Matchers
 } // namespace Catch
-
-#endif // CATCH_CONFIG_DISABLE_MATCHERS