refactor(miniooni): divide et impera (#912)

This diff splits miniooni's implementation in smaller and more
easily tractable blocks ahead of future refactoring.

I'm trying to make `miniooni oonirun -i URL` as possible as
`miniooni -i URL oonirun`, because users typically expect this
kind of flexibity from modern Unix commands.

Part of https://github.com/ooni/probe/issues/2184

internal/cmd/miniooni/logging.go

@@ -0,0 +1,35 @@
+package main
+
+//
+// Logging functionality
+//
+
+import (
+	"fmt"
+	"io"
+	"time"
+
+	"github.com/apex/log"
+)
+
+// logStartTime is the time when we started logging
+var logStartTime = time.Now()
+
+// logHandler implements the log handler required by github.com/apex/log
+type logHandler struct {
+	// Writer is the underlying writer
+	io.Writer
+}
+
+var _ log.Handler = &logHandler{}
+
+// HandleLog implements log.Handler
+func (h *logHandler) HandleLog(e *log.Entry) (err error) {
+	s := fmt.Sprintf("[%14.6f] <%s> %s", time.Since(logStartTime).Seconds(), e.Level, e.Message)
+	if len(e.Fields) > 0 {
+		s += fmt.Sprintf(": %+v", e.Fields)
+	}
+	s += "\n"
+	_, err = h.Writer.Write([]byte(s))
+	return
+}