Make the logger customisable

Add a new `--log-format` cli argument to change the format of the logs.
The possible values are
- raw (the default one for old-style commands)
- bar (the default one for new-style commands)
- bar-with-logs (equivalent to `--print-build-logs`)
- internal-json (the internal machine-readable json format)

src/libutil/logging.hh

@@ -63,6 +63,8 @@ public:
 
     virtual ~Logger() { }
 
+    virtual void stop() { };
+
     virtual void log(Verbosity lvl, const FormatOrString & fs) = 0;
 
     void log(const FormatOrString & fs)
@@ -141,7 +143,7 @@ struct PushActivity
 
 extern Logger * logger;
 
-Logger * makeDefaultLogger();
+Logger * makeSimpleLogger();
 
 Logger * makeJSONLogger(Logger & prevLogger);