Add support for multiple parallel reporters

This requires a bunch of different changes across the reporter
subsystem.

* We need to handle multiple reporters and their differing
  preferences in `ListeningReporter`, e.g. what to do when
  we mix reporters that capture and don't capture stdout.
* We need to change how the reporter is given output and
  how we parse reporter's output destination from CLI.
* Approval tests need to handle multireporter option

src/catch2/internal/catch_console_colour.cpp

@@ -159,7 +159,7 @@ namespace {
         void setColour( const char* _escapeCode ) {
             // The escape sequence must be flushed to console, otherwise if
             // stdin and stderr are intermixed, we'd get accidentally coloured output.
-            getCurrentContext().getConfig()->stream()
+            getCurrentContext().getConfig()->defaultStream()
                 << '\033' << _escapeCode << std::flush;
         }
     };