Refactor colour handling to prepare for per-reporter colour modes

This includes always compiling the ANSI and None colour
implementations, as they don't need to touch any platform
specific APIs, and removing their respective compile-time
configuration options.

Because the Win32 colour implementation requires Win32-specific
APIs, it is still hidden behind a compile-time toggle,
`CATCH_CONFIG_COLOUR_WIN32` (renamed from `..._COLOUR_WINDOWS`).

The commandline options for colours were also changed. The
option now uses different name, and allows to select between
different implementations, rather than changing whether
the compiled-in colour implementation is used through
"yes/no/default" options.

src/catch2/catch_session.cpp

@@ -152,7 +152,8 @@ namespace Catch {
 
             m_startupExceptions = true;
             auto errStream = makeStream( "%stderr" );
-            auto colourImpl = makeColourImpl( &config(), errStream.get() );
+            auto colourImpl = makeColourImpl(
+                ColourMode::PlatformDefault, errStream.get() );
             auto guard = colourImpl->guardColour( Colour::Red );
             errStream->stream() << "Errors occurred during startup!" << '\n';
             // iterate over all exceptions and notify user
@@ -197,7 +198,7 @@ namespace Catch {
             config();
             getCurrentMutableContext().setConfig(m_config.get());
             auto errStream = makeStream( "%stderr" );
-            auto colour = makeColourImpl( &config(), errStream.get() );
+            auto colour = makeColourImpl( ColourMode::PlatformDefault, errStream.get() );
 
             errStream->stream()
                 << colour->guardColour( Colour::Red )