allow set ssh options for sshkit backend

you can pass shh options for backend eg

      SSHKit.configure do |sshkit|
         sshkit.backend.configure do |backend|
            backend.ssh_options = {
               keys: %w(/home/user/.ssh/id_rsa),
               forward_agent: false,
               auth_methods: %w(publickey password)
            }
         end
      end
Merge pull request #22 from korin/ssh-options

Allow set ssh options for sshkit backend.