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_reporter_registry.cpp

@@ -36,11 +36,11 @@ namespace Catch {
     ReporterRegistry::~ReporterRegistry() = default;
 
 
-    IStreamingReporterPtr ReporterRegistry::create( std::string const& name, IConfig const* config ) const {
+    IStreamingReporterPtr ReporterRegistry::create( std::string const& name, ReporterConfig const& config ) const {
         auto it =  m_factories.find( name );
         if( it == m_factories.end() )
             return nullptr;
-        return it->second->create( ReporterConfig( config ) );
+        return it->second->create( config );
     }
 
     void ReporterRegistry::registerReporter( std::string const& name, IReporterFactoryPtr factory ) {