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_commandline.hpp

@@ -170,6 +170,10 @@ namespace Catch {
             .describe( "set a specific seed for random numbers" )
             .bind( &setRngSeed, "'time'|number" );
 
+        cli["--force-colour"]
+            .describe( "force colourised output" )
+            .bind( &ConfigData::forceColour );
+
         return cli;
     }