Ability to install without using npm -g
### Current Situation

Right now, all of the installation instructions require giving the homebridge user superuser permissions so that it can install via `npm -g`, and the automatic upgrade functionality within homebridge itself also assumes `npm -g`. However, this is fairly unsafe especially if you're running homebridge on a machine which runs multiple purposes, and it's also unnecessary as packages can simply be installed into the application's own little workspace/sandbox by installing without `-g`. In my own setup I run homebridge in this way, and just have to manually upgrade my packages from the command line instead.

### Proposed Change

Make it an option to use `npm install` instead of `npm -g install` to allow better security practices on a multiuser system.

### Additional Context

_No response_