[Github] Shared tokens, expiration

a couple of ideas, around github expiring tokens

1. I now find i have several targets sharing the same `github.token`. Would it be possible to use named tokens such as the following, so that updating a token is easier:

```ini
[github_abc]
service = github
github.token = $token3

[github_def]
service = github
github.token = $token3

[general.tokens]
token3=ghp_123456
```

2. according to [this page](https://github.blog/changelog/2021-07-26-expiration-options-for-personal-access-tokens/) there is a header `GitHub-Authentication-Token-Expiration` giving the expiration date. Could the tool give this feedback back on the command line? it should be collated so that only one message is given per token, even if the token is used multiple times.