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.

tests/CMakeLists.txt

@@ -473,6 +473,17 @@ set_tests_properties("Reporters::DashAsLocationInReporterSpecSendsOutputToStdout
     PASS_REGULAR_EXPRESSION "All tests passed \\(5 assertions in 1 test case\\)"
 )
 
+add_test(NAME "Colours::ColourModeCanBeExplicitlySetToAnsi"
+  COMMAND
+    $<TARGET_FILE:SelfTest> "Factorials are computed"
+      --reporter console
+      --colour-mode ansi
+)
+set_tests_properties("Colours::ColourModeCanBeExplicitlySetToAnsi"
+  PROPERTIES
+    PASS_REGULAR_EXPRESSION "\\[1\;32mAll tests passed"
+)
+
 if (CATCH_ENABLE_CONFIGURE_TESTS)
     add_test(NAME "CMakeConfig::DefaultReporter"
       COMMAND