✨(schema): adds support for setting config options globally

[ahrjarrett]

new features

This PR adds the ability for users to set configuration options globally. This is convenient when you have certain cross-cutting requirements, and would like them to be enforced/consistent across the codebase.

example

To set a global configuration option, import configure from the @traversable/schema package.

Note: configuration is optional -- if the defaults suit you, this PR doesn't add any additional hoops to jump through.

Note: successive calls to configure will continue to update the global config object.

import { configure, t } from '@traversable/schema'

const schema = t.object({
  a: t.number,
  b: t.optional(t.string),
})

// before:
schema({
  a: 0,
  b: undefined,
}) // ✅

// with this options, all object and tuple schemas will perform 
// an extra check to make sure that optional properties are either 
// present, or missing altogether, but never present + undefined

configure({ schema: { optionalTreatment: 'exactOptional' } })

// after:
schema({
  a: 0,
  b: undefined,
}) // 🚫 

// Yay, this is the behavior we want