Add support for ignoring keys

[mafredri]

Hi,

I wanted to exclude one key from GitHub that I don't use for remote server SSH, here's a simple implementation that does string matching on a per-line basis.

This would allow --ignore ssh-ed25519 to ignore all ed25519 keys (for whatever reason 🙄) and --ignore "ssh-ed25519 AAAA...." to ignore a specific key.

Let me know if it's not a good fit or you want me to change something 😄.