Add sources/current/targets logic to morph

[dmnsgn]

• Update gltf-morph.js example (do you think we can remove it now @vorg ? As the gltf example will cover morph examples.)
• Update gltf-build to enable multiple attributes to morph
• Update Morph component to handle multiple attributes and a .current property (init and update)

Fix #175

[vorg]

Yes we can remove morph example.