Keep the camera position when switching to inspector (fix #607)

[vincentfretin]

Keep the camera position and rotation when opening the inspector.

I have this change for quite some times in my project. When I use the default inspector build in other projects I am so frustrated that the camera position is not kept while opening the inspector.
It seems I am not the only one that wanted this behavior.

This closes #607

[vincentfretin]

But it should be an option somehow. I want that behavior when I am on a scene and I open the inspector.
But I don't want that behavior when opening directly the inspector and want the perspective camera to be at the specified position

aframe-inspector/src/lib/cameras.js

Lines 45 to 46 in 575c910

	perspectiveCamera.position.set(0, 1.6, 2);
	perspectiveCamera.lookAt(new THREE.Vector3(0, 1.6, -1));

Those positions should also be configurable, with the ortho cameras as well. I'm writing some configurators where I want different camera positions.