feat(module): support module.rules for webpack 2

[outsideris]

close #136

I'm not sure it's a correct way to support module.rules because it's my first time with joi.

[kentcdodds]

Thanks for the PR! I realize you probably put a lot of work into it. But I'm afraid you've missed an important note in the README:

Note: webpack v2 has built-in validation for configuration. Due to this, webpack-validator is unlikely to make significant changes. While pull requests will be reviewed and can be merged, project maintainers are unlikely to put a lot of much effort into the maintenance of the project.

I don't think that we'll merge your PR :-/

[outsideris]

@kentcdodds Oops, I missed it. Thank you for notifying me.