Exports Config from metal-soy

[mthadley]

This exports Config from metal-soy in the same way that we do in metal-jsx.

Also, I wonder if going forward we should encourage using Config instead of doing something like validator: core.isString. Config seems to be more concise, and provide better feedback (in the form of warnings) when you provide invalid props. It's also more expressive and self-documenting when you need to validate enumerations (oneOf).

[eduardolundgren]

For consistency I think it's a good addition. I will merge it, let me know if anyone has any concern.

/cc @mairatma