Error or warning when using an expired certificate?

まだ誰も着手していません。

Hello! I'm wondering if it would be possible to configure smimesign to error out (or at least display a warning) if the certificate you are using to sign commits has expired. I am trying to implement a system for git signing with shorter-lived developer certificates (30 days) and it would be really helpful if a warning or error message would let developers know that their certificate has expired and they should request a new one.

Currently smimesign will go ahead and sign commits with expired certificates with no output, but then verification fails:

```
$ git log --show-signature
commit 923459dd4798833eb56f0f376fcd6d0b3fc86e04 (HEAD -> master)
failed to verify signature: x509: certificate has expired or is not yet valid: current time 2021-10-01T13:56:16-05:00 is after 2021-10-01T18:54:21Z
```

I know we're limited in what git passes to smimesign so i'd be fine to use a wrapper script or command, but would need a command line flag to indicate it should exit with error if the signing certificate is expired.

Let me know your thoughts and I'd be happy to open a PR!

まず smimesign の署名コマンドと証明書処理の経路を特定し、次にコマンドラインオプションの定義方法を確認します。署名前に証明書の有効期限切れを検出できる箇所を特定し、要求された動作を警告にするかエラーを返すようにするかを決定します。期限切れの署名証明書が選択した可視的な応答を生成し、有効な証明書が引き続き通常どおり署名できれば完了です。

索引モデルが issue の本文から書いたものです。