Minor bugfixes: add lightColor, allow non-CSM light sources

[mjurczyk]

Small bugfixes for 2 long-standing issues:

Solves #13

Changes:

• Add lightColor to CSM constructor
• Add to docs

Solves #10

Changes:

• Allow usage with other light & shadow sources (limit shader loop to CSM_CASCADES, and loop through the reminder of directional lights normally)
• Instead of appending lights using Object3D#add, prepend them to the parent as first light sources on the children's list

(left - CSM directional light; right - another directional light; no more WebGL errors thrown on directionalLights loop)

[StrandedKitty]

Hi, thank you for this PR! I like your solution for differentiating CSM light sources from regular light sources in the shader.

I've committed some minor fixes, if everything looks okay to you then I will merge it.

[mjurczyk]

LGTM