Improve logging and error handling

- enable logging by default: debug was a bad name - it's error logging that I
  just want to hide in tests
- don't panic (all the time)
- use a logger. this allows us to add more information - like the path of the
  parsed file!

org/html_test.go

@@ -9,7 +9,7 @@ func TestHTMLWriter(t *testing.T) {
 	for _, path := range orgTestFiles() {
 		expected := fileString(path[:len(path)-len(".org")] + ".html")
 		reader, writer := strings.NewReader(fileString(path)), NewHTMLWriter()
-		actual, err := NewDocument().SetPath(path).Parse(reader).Write(writer)
+		actual, err := NewDocument().Silent().SetPath(path).Parse(reader).Write(writer)
 		if err != nil {
 			t.Errorf("%s\n got error: %s", path, err)
 			continue