Allow setting "with credentials" option with custom element attribute

[jasonford]

Thanks for the very useful module! I'm running into an issue where my site needs to make requests having "with credentials" set to true. It looks like pdfjs supports this, but I don't see a means to set that at the custom element level with this package.

Happy to submit a PR to this repo for the feature, but I might need to ask a couple of questions to make sure I'm doing it right, and this type of thing might be easier to do rather than help do.

Let me know if this feature sounds good, and if starting to work on a PR sounds good.

[alekswebnet]

Hi @jasonford
After the quick research, I didn't find a possible way to do it in PDF.js viewer. Maybe it is worth to dig deeper. Anyway, you are welcome to suggest your variants or submit a PR.