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_console_colour_impl.hpp

@@ -143,7 +143,8 @@ namespace {
     };
 
     IColourImpl* platformColourInstance() {
-        return isatty(STDOUT_FILENO)
+        Ptr<IConfig const> config = getCurrentContext().getConfig();
+        return (config && config->forceColour()) || isatty(STDOUT_FILENO)
             ? PosixColourImpl::instance()
             : NoColourImpl::instance();
     }