Calling `critical.generate` lots of times in parallel seems to cause MaxListenersExceededWarning

I am the author of the [eleventy-critical-css](https://github.com/gregives/eleventy-critical-css) Eleventy plugin which calls `critical.generate` for every HTML file outputted by Eleventy. [An issue](https://github.com/gregives/eleventy-critical-css/issues/7) was filed on eleventy-critical-css saying that the plugin was causing a MaxListenersExceededWarning.

When the plugin is used with larger Eleventy sites, `critical.generate` is called lots of times in parallel and it causes a MaxListenersExceededWarning. Here is the stack trace:

```
(node:12968) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 SIGINT listeners added to [process]. Use emitter.setMaxListeners() to increase limit
at _addListener (events.js:475:17)
at process.addListener (events.js:497:10)
at module.exports (D:\Projects\critical-issue-528-reproduction\node_modules\penthouse\lib\index.js:160:11)
at D:\Projects\critical-issue-528-reproduction\node_modules\critical\src\core.js:75:20
at D:\Projects\critical-issue-528-reproduction\node_modules\p-all\index.js:4:67
at D:\Projects\critical-issue-528-reproduction\node_modules\p-map\index.js:57:28
```

[This repository](https://github.com/atlas-bi/website) was added to the issue on eleventy-critical-css which successfully reproduces the problem. ~~If I have time, I will create a simpler reproduction, probably just by calling `critical.generate` lots of times in parallel.~~ **See new simpler repository below.**

I am unsure if this is a problem with Critical or with Penthouse, let me know if you want me to raise an issue in the Penthouse repository instead. Thanks for your help!