component-build --use flag to specify plugins

This allows builder.js plugins/middleware to be specified as part of the component-build cli. For example:

```bash
$ component build --use jade-builder
```

The example [jade-builder](https://github.com/nickjackson/jade-builder) is a module I just put together from an example in the builder repo.

A relative or absolute path can be specified, or you can specify a module name that would need to be pre-installed in the users project `./node_modules` directory. Multiple plugins can be specified by separating the files with a comma.

https://github.com/component/component/issues/268