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.

CMake/CatchConfigOptions.cmake

@@ -26,6 +26,7 @@ endmacro()
 
 set(_OverridableOptions
   "ANDROID_LOGWRITE"
+  "COLOUR_WIN32"
   "COUNTER"
   "CPP11_TO_STRING"
   "CPP17_BYTE"
@@ -45,9 +46,6 @@ foreach(OptionName ${_OverridableOptions})
 endforeach()
 
 set(_OtherConfigOptions
-  "COLOUR_ANSI"
-  "COLOUR_NONE"
-  "COLOUR_WINDOWS"
   "DISABLE_EXCEPTIONS"
   "DISABLE_EXCEPTIONS_CUSTOM_HANDLER"
   "DISABLE"