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_config.cpp

@@ -138,7 +138,7 @@ namespace Catch {
     uint32_t Config::rngSeed() const                   { return m_data.rngSeed; }
     unsigned int Config::shardCount() const            { return m_data.shardCount; }
     unsigned int Config::shardIndex() const            { return m_data.shardIndex; }
-    UseColour Config::useColour() const                { return m_data.useColour; }
+    ColourMode Config::colourMode() const              { return m_data.colourMode; }
     bool Config::shouldDebugBreak() const              { return m_data.shouldDebugBreak; }
     int Config::abortAfter() const                     { return m_data.abortAfter; }
     bool Config::showInvisibles() const                { return m_data.showInvisibles; }