Store parsed DER certificates for matching entries

For convenience.  For example, now one can debug issues like this:

    openssl x509 -inform DER -in /path/to/crt.der -text -noout

1 files changed, 5 insertions, 2 deletions

diff --git a/pkg/storage/index/index.go b/pkg/storage/index/index.go
index 36a9334..0056565 100644
--- a/pkg/storage/index/index.go
+++ b/pkg/storage/index/index.go
@@ -78,8 +78,11 @@ func (ix *Index) AddEntries(logID [sha256.Size]byte, entries []monitor.LogEntry)
 		if !ix.mem.addEntry(crtID, path) {
 			return nil // duplicate
 		}
-
-		return ioutil.CommitJSON(path, entry)
+		if err := ioutil.CommitJSON(path, entry); err != nil {
+			return err
+		}
+		path = fmt.Sprintf("%s/%x-%d.der", ix.cfg.MatchDirectory, logID[:], entry.LeafIndex)
+		return ioutil.CommitData(path, crt.Raw)
 	}
 
 	for _, entry := range entries {