SSB-Keyfile

Keyfile operations for ssb: Read keys from a keyfile and create new keyfiles.

Creates and reads keyfiles that are compatible with the js ssb implementation.

```rust let keypair = ssbkeyfile::generateat_path("/path/to/secret")?;

let keypair = ssbkeyfile::readfrom_path("/path/to/secret")?; ```

cargo install ssb-keyfile ssb-keyfile new --path ~/.ssb-foo/secret ssb-keyfile new --path ~/.ssb-bar/secret --secret XQfgelZViM6npy...