When detecting the format, detect its version as well

There is no need to detect the format and then the version when both can be
done at the same time.

Add a benchmark as well, on large and small atom and rss files.

internal/reader/parser/format.go

@@ -21,12 +21,12 @@ const (
 )
 
 // DetectFeedFormat tries to guess the feed format from input data.
-func DetectFeedFormat(r io.ReadSeeker) string {
+func DetectFeedFormat(r io.ReadSeeker) (string, string) {
 	data := make([]byte, 512)
 	r.Read(data)
 
 	if bytes.HasPrefix(bytes.TrimSpace(data), []byte("{")) {
-		return FormatJSON
+		return FormatJSON, ""
 	}
 
 	r.Seek(0, io.SeekStart)
@@ -41,14 +41,19 @@ func DetectFeedFormat(r io.ReadSeeker) string {
 		if element, ok := token.(xml.StartElement); ok {
 			switch element.Name.Local {
 			case "rss":
-				return FormatRSS
+				return FormatRSS, ""
 			case "feed":
-				return FormatAtom
+				for _, attr := range element.Attr {
+					if attr.Name.Local == "version" && attr.Value == "0.3" {
+						return FormatAtom, "0.3"
+					}
+				}
+				return FormatAtom, "1.0"
 			case "RDF":
-				return FormatRDF
+				return FormatRDF, ""
 			}
 		}
 	}
 
-	return FormatUnknown
+	return FormatUnknown, ""
 }