Add userAgent option

[tamorim]

Hello! First of all, thanks to all the contributors to this project. 💯

I'm stepping in some edge case here, I guess.
The app I'm working on right now is adaptive without different urls for mobile, so I pass the userAgent option to penthouse where he applies it just fine.
Since I only provide the src (no css files or strings) as a remote url, critical goes and fetches the html file and extracts the css files from the html.
The problem is that when critical goes to fetch the html file, since it does't use any custom user agent, it retrieves the "default" page which doesn't contain the desired css file for that case, generating "wrong" critical css output.

What do you guys think? Is it possible to add a option like this?
I thought that it would be useful and serve as an abstraction to the penthouse.userAgent as well.
I'm willing to open a PR for it if that kind of option is desired.

[raxbg]

I agree completely. In fact I have seen many sites that behave like that. I made a small change to the file-helper.js which allows you to specify the user agent through an environment USER_AGENT variable. Please see the referenced pull request.

I hope that we will get official support for this in the next release.

[tamorim]

@raxbg I hope that you don't mind that I've opened a PR myself too.
I'm already rolling with a critical fork with the userAgent option for some weeks at my job.
Seeing your comment gave me some strength to rebase and add a test for it haha, I hope that you like the solution too!
I'm open to any feedback

[raxbg]

@tamorim Thanks for sharing this. It's exactly what was needed 👍