That would be an excellent feature and years ago I had a laptop that had a simple plastic sliding piece that would cover the camera lens. Later versions eliminated this piece (it probably broke off too frequently).

I have taped a piece of aluminum foil over every webcam on my laptops/iMac with a large piece of packing tape (making it possible to pull it aside when I do rather frequent video conferences).

I get dismissed as paranoid (or the next question that's asked is "what do you do in front of your computer!"). What they don't understand is that years ago I wrote software for a building access kiosk that involved a one-way video chat, followed with a photograph taken of the visitor. I used an open source library for interacting with the camera. When video was running, the light was on. I had the worst time getting the camera to take a picture, but after a bit of tinkering (read: shooting bullets randomly), I had it working. It was my coworker that noticed the light didn't come on when the picture was being taken. I had done something wrong (so wrong that every few pictures, the camera would stop working and wouldn't recover until the entire machine was rebooted). Though I've been writing software professionally for twenty years or so, I had never wrote code to interact with a web cam and yet had somehow managed to stumble upon this entirely by accident. It gave me the willies, so from that point forward, I covered my cameras with a bit of ugly aluminum foil.

I've never really worried about the microphone, but it would be nice to have some kind of hardware control to disable devices that can be used to record in that manner.