Timeout when generating from a Storybook story
When generating from a local Storybook server, you receive a request timeout error:

```
$ html-sketchapp --url 'http://localhost:6006/iframe.html?selectedKind=Sketch&selectedStory=Export' --out-dir dist
{ TimeoutError: Navigation Timeout Exceeded: 30000ms exceeded
    at Promise.then (buildkite/frontend/node_modules/puppeteer/lib/NavigatorWatcher.js:94:21) name: 'TimeoutError' }
```

Storybook keeps a webpack HMR event-stream socket open. Switching from `networkidle0` to `networkidle2` fixes it.

Given `networkidle2` is the recommended default event to use these days anyhow, perhaps you'd be open to a PR that replaces `networkidle0` with `networkidle2`?

Aside from this one glitch, this works perfectly w/ Storybook — you just create a story with a big Sketch export sheet, and point the CLI directly at it 👌🏼