Add support for more text types

[felixmariotto]

It would be great if the library supported optional alternative text types for alternative use-cases.

For instance, one can imagine an instancedText based on THREE.InstancedMesh, to support fast-changing text.

One can also imagine a vectorText to support complex font that cannot be well rendered with MSDF, or that must be rendered very large with exactitude.

The architecture to support alternative text types is ready in TextManager

related : #11