Support -p flag (public key file)

It's missing from minitrust but exists in original minisign.

```sh
$ minisign -h
Usage:
[...]
minisign -V [-H] [-x sig_file] [-p pubkey_file | -P pubkey] [-o] [-q] -m file

[...]
-p <pubkey_file>  public key file (default: ./minisign.pub)
-P <pubkey>       public key, as a base64 string
[...]
```