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

tests/SelfTest/Baselines/default.sw.multi.approved.txt

@@ -0,0 +1,8 @@
+This would not be caught previously
+Nor would this
+A string sent directly to stdout
+A string sent directly to stderr
+A string sent to stderr via clog
+Message from section one
+Message from section two
+loose text artifact