add option to run commands

[pierredup]

It would be nice to add an option in the yaml config to run commands.
E.G when packaging a Symfony2 application, you might want to do something like

commands:
    - php app/console assets:install -e=prod

Or to get the latest version of dependencies before packaging

- php composer.phar update

Or

- git submodule update

If my ruby wasn't so rusty, I would send a PR

[nirix]

Good idea, shouldn't take long to implement.