put use_private_dns in parentheses refs #10
Delete 'pry' for debug refs #10
use 'ssh_options' in dotfile refs #10
update README.md refs #10

- add --use-private-dns option description
- add ssh options Description
 - include @frsyuki 's additions
  - https://github.com/frsyuki/ec2ssh/blob/226cff4765a4ee39303a51f3d086f525ae00c2a8/README.md