Media command regenerate all of one by ID

First part of the media command as spoken in #88
Scribu mentions @benmay ported the coded from @Viper007Bond to wp-cli.
Took the time to integrate the whole thing.

`wp media regenerate --id=xx` regenerates only that image  
`wp media regenerate` default regenerates all images.

Should there be a extra param `--all` to explicity start the regenrating and add a check like

`if ( !$id && !$all)
  
ext(1);