Add interface to Command class

[HeathNaylor]

Apologies if this is sloppy in any way, this is my first contribution to a Symfony project component. The scope here is very simple, just adding an interface to the Command class.

I found documentation about submitting patches but it was specific to the Symfony framework, so I am a little unclear if I need to do anything else. There really isn't any test coverage that should need to be added.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/symfony

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the master branch.