Public key authentication key files path as configurable option

Key files path for SFTP public key authentication was made configurable

git config git-ftp.sftp-key ~/.ssh/id_rsa
git config git-ftp.sftp-public-key ~/.ssh/id_rsa

implements resmo/git-ftp/#184
Renamed the parameter names to match curl options

Renamed --sftp-key to --key
Renamed --sftp-public-key to --pubkey
Replaced equality operator '==' with '='
Updated man page to include the new options.
Merge pull request #6 from JoyceBabu/develop

Public key authentication key files path as configurable option