Validate configuration against schema

[lerebear]

To help users configure this tool correctly, we should validate user-supplied configuration against our schema. This well help us catch and report structural errors more quickly.

I think this could have helped with #16, which turned out not to be a bug in this library, but rather a misplaced configuration key (the optIns key was accidentally nested under the sizeup key, where it should instead have appeared at the top level). If we encounter unexpected/misplaced keys during the validation process, we should warn on them, whereas if we find invalid values for a key we should fail the workflow.