Public key authentication key files path as configurable option

Key files path for SFTP public key authentication was made configurable

    git config git-ftp.key ~/.ssh/id_rsa
    git config git-ftp.pubkey ~/.ssh/id_rsa

The option may also be specified at run time

    git ftp push --key ~/.ssh/id_rsa --pubkey ~/.ssh/id_rsa.pub

If the public key is not specified, then the private key name with `.pub` extension is used.

implements resmo/git-ftp/#184