Add --force-colour option to force colour output.

Adding a --force-colour option to force colour output on POSIX systems,
provided a debugger is not attached.

This allows for Catch to output colours even if STDOUT is not a tty,
which can be the case when the test executable is being spawned by a
parent process (e.g. CMake's ctest).

include/internal/catch_interfaces_config.h

@@ -56,6 +56,7 @@ namespace Catch {
         virtual TestSpec const& testSpec() const = 0;
         virtual RunTests::InWhatOrder runOrder() const = 0;
         virtual unsigned int rngSeed() const = 0;
+        virtual bool forceColour() const = 0;
     };
 }