Create directories if they don't exist

This release fixes a bug when generating key pairs or signing a file with a custom signature file (instead of <file>.minisig).
If the key pair or signature should have been written to a file in a directory that does not exist the key generation / signing used to fail.
minisign just didn't create any non-existing directory.

Now, minisign creates any non-existing directory when generating a key pair or signing a file with a custom signature file.