Bumblebee config to enable multiple monitors

Added configurations to `bumblebee` package to easy multiple monitors on Optimus
machines.

The behaviour of the default `bumblebee` package hasn't change, so this change
is backwards compatible. Users who want to connect a monitor to their discrete
card should use the package `bumblebee_display` instead.

Also added new configuration option to nixos bumblebee module:

```
hardware.bumblebee.connectDisplay = true
```

will enable the new configuration, but the default is still false.