Allow for HTML injection

[kvizdos]

Hello!

I'm really loving this proxy system, but I was wondering if it was possible to allow for custom HTML injections onto proxied sites? Let's say you are proxying to a web-based VSCode, but for uniformity, you want to inject a menu object that users can click to navigate either back to a menu of other services or just to a certain service.

As far as I know, you can't do it yet, but would anyone else like this feature? I think it'd be useful for a couple of situations!