feat(report) : Differences between vulnerability patched items (#1157)

* add plusDiff() and minusDiff()
* add plusDiff minusDiff test

Co-authored-by: Kota Kanbe <kotakanbe@gmail.com>

models/scanresults.go

@@ -352,7 +352,7 @@ func (r ScanResult) FormatTextReportHeader() string {
 		pkgs = fmt.Sprintf("%s, %d libs", pkgs, r.LibraryScanners.Total())
 	}
 
-	return fmt.Sprintf("%s\n%s\n%s, %s, %s, %s, %s\n%s\n",
+	return fmt.Sprintf("%s\n%s\n%s\n%s, %s, %s, %s\n%s\n",
 		r.ServerInfo(),
 		buf.String(),
 		r.ScannedCves.FormatCveSummary(),