Add options to set scene background

[leakec]

• [x] Add options to the parser to set the scene background.
• [x] Can probably add this in a general way so we can add more properties to the scene, e.g.,

[leakec]

Some skybox textures can be found here

[leakec]

Closing as this was added in https://github.com/leakec/sihm/commit/96f220bcc4b9f31b674d4ce53f9846841aa38c1d