CLI option to reset cache but not storage APIs

Hi everyone :)

I've started using lighthouse for one of our large scale Angular 2/4 apps and it's working really nicely. Thank you!

The app has authentication, cookie-layers etc... So I needed to run lighthouse with `chrome-debug` and use `--disable-storage-reset` which does [[link](https://github.com/GoogleChrome/lighthouse/blob/master/docs/readme.md#testing-on-a-site-with-authentication)]:
```
Disable clearing the browser cache and other storage APIs before a run
```
Incidentally I was hoping that `--disable-storage-reset` would only disable a reset for Cookies, localstorage and the other APIs and that the cache on the other hand would still be reset before every run.

That behavior is the one I'm most interested in since it's a very common case that users will access new areas (no content in browser cache) of the app but will have already logged in. Currently I can't test that.

So I'd recommend another CLI option that does just that.

What do you think?

Best, Jon