Enhancement: Allow additional parameters for sky

[dsinni]

I was working on a project not long ago, and I needed the sky to also be visible from the exterior (space), which actually looked great after I used something like the custom code, below:

'''
AFRAME.registerComponent('planet-sky', {

init: function () {

var scene = this.el.sceneEl;
var sky = scene.querySelector('a-sky');

sky.setAttribute('material', 'side', 'double');
sky.setAttribute('theta-length', 180);

}

});
'''

It took some poking around at first to achieve this, so I was thinking it would be great if it were possible to easily set sky parameters such as geometry.thetaLength and material.side for a full sphere that can also be seen from the exterior.

Of course, radius would also be great, which I mentioned in #45.

Just a thought. Keep up the great work!