Media command regenerate all or 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.

1. `wp media regenerate` default regenerates all images.

Should there be a extra param `--all` to explicity start the regenrating and add a check like this:
```
if ( !$id && !$all)
  error('Please provide proper args');
  exit(1);
endif
```

2. `wp media regenerate --id=xx` regenerates only that image