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/interfaces/catch_interfaces_reporter.cpp

@@ -20,9 +20,6 @@
 
 namespace Catch {
 
-    ReporterConfig::ReporterConfig( IConfig const* _fullConfig )
-    :   m_stream( &_fullConfig->stream() ), m_fullConfig( _fullConfig ) {}
-
     ReporterConfig::ReporterConfig( IConfig const* _fullConfig, std::ostream& _stream )
     :   m_stream( &_stream ), m_fullConfig( _fullConfig ) {}