Add meta tag support for custom skybox

[mentatbot[bot]]

This update introduces a new meta tag for specifying a custom skybox URL. The meta tag allows users to set a cube map URL, which is then incorporated into the render loop using native three.js functionality. The existing skybox behavior is disabled when a custom skybox is specified. This enhancement addresses issue #60.

Closes #60

[gfodor]

A good start - please review dom-utils.js for how to properly manage the meta tag state, particularly note how the terrain meta tags are dealt with since this would largely fall under the same umbrella.

[gfodor]

@MentatBot