Get something basic working

[bvssvni]

See https://github.com/PistonDevelopers/shaders/issues/7

[bvssvni]

Could rewrite the code from the prototype with an upgraded version of wavefront_obj Prototype: https://github.com/bvssvni/ld30/

This might be redesigned later to support more formats/different libraries.

[bvssvni]

• [x] Add Geometry::add_geometry
• [x] Add Object::add_object
• [x] Add Model::add_model

[bvssvni]

The index buffer can have multiple parents, and it might be possible to synchronize custom arrays with range buffers.