Adding ignore option to ignore a list of elements during the sync

I switched over `exec` to avoid the escaping of the arguments.
Also, it forced to make the `--rsh|-e` flag more explicit.

This is an example or `_config.yml` I have for my own website:
```yml
deploy:
  type: rsync
  host: oncletom
  user: oncletom
  root: ~/
  delete: true
  exclude: [.ssh, .bash*, .htaccess, images, talks]
```