feat: add config option for uniform lighting

[MShinkle]

Discussed in #560, this adds a simple option to enforce uniform lighting (no shadows or specularity) within pycortex webgl viewers. This is turned on by adding uniform_illumination = true to the user config file.

This option is set to false by default, so lighting should remain as it currently is unless this option is changed to true.

[mvdoc]

I understand this can be too much, but I'll ask anyway because why not ;)
Do you think you could add a button in the UI to enable/disable this option?
I think in this way we'll be able to expose this flag to cortex.export.save_view too, so it will make it easy to automate the generation of figures with uniform lighting.

[MShinkle]

Good idea! I haven't tested it with cortex.export.save_view, but I just added a commit that adds a toggle box for this illumination option in the UI.

[mvdoc]

Awesome, thank you!