Make --debug set clean-webpack-plugin's 'verbose' option (#392)
Make --debug set clean-webpack-plugin's 'verbose' option (#392)

This gives output of form:

```
clean-webpack-plugin: C:\Users\Ed\src\treeherder\build has been removed.
```

See:
https://github.com/johnagan/clean-webpack-plugin#options-and-defaults-optional

Refs #389.
edmorley/neutrino-dev,mozilla-neutrino/neutrino-dev,mozilla-neutrino/neutrino-dev,eliperelman/neutrino-dev,edmorley/neutrino-dev