Multiple file input / cross-input validation

[samhh]

How will the locale be supplied or inferred?

[samhh]

If we decide to go this route we may consider a change in JSON structure:

{
  "locale": "en-US",
  "messages": {
    "header": {
      "message": "A very fun header"
    }
  }
}

Glob input would be fairly flexible, but that'd imply assuming sibling output, which is less flexible than stdout.

[samhh]

This ties into #1. In both cases we can no longer output to stdout and must instead decide upon a new output strategy.