Pluggable target node types

[brianchirls]

Similar to how you can create plugins for Source types
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WebGL canvas targets will remain built-in. Other types like FBO/Texture and 2D canvas, may be moved out into plugins. The goal is to eventually build target plugins for other frameworks like Three.js, Babylon.js, so output from Seriously.js can be used as textures for 3D scenes. Those will be filed under separate issues.