Add headDummy parameter

[feiss]

..or head or showDefaultHead or whatever, so users can hide default pink box for the head with headDummy:false and use another model for the head instead

[feiss]

Oh, digging in the code I see that if the <a-camera> has already a model attached via some component, it does not add the cube.

For example:
<a-camera gltf-model="#model"></a-camera>

makes sense, and it works for me now, but I guess somebody would like to add models via children nodes of the camera (?)