Add {:coercion :select-schema} option

[aneilbaboo]

It would be incredibly handy to be able to filter out unwanted keys from an object based on the schema rather than reporting errors. I noticed the select-schema function in schema-tools, but I'm not quite sure how to use that to create my own Coercion.

I might create a Coercion and submit a PR if someone can guide me in how to do it.