Feature Request: Support users with multiple identities.

Allow users being able to add multiple keys for the same user. Basically, check against a list of valid keys rather than a single key. The data would look like for example:

`data/keys/user1`:
```
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICsfZlzGJSe6B5q5mEp9E2blI/xW7mW34h1xdHxFUQHs user1@sdslabs.co
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICUKyjr6icfamUc4l7tPFr82VgSf373rciLGcEGPYirv user1@sdslabs.co
```