Design a public API for native apps

Just like `npm` and command-line version, the validator needs a convenient and well-documented public API for native apps. Open questions:
- [x] How to pass a list of ignored issues and a map of overridden severities? Could apps pass a YAML string?
- [x] Validation report is represented as JSON document. Could the validator just return it as a string?
- [ ] How should the validator get asset's data? Should it access files directly (like command-line version) or expect data from calling apps (like `npm` build)? How external resources should be accessed?

Start by reviewing the existing npm and command-line interfaces and the checked API decisions in this issue. Resolve how calling apps provide asset data and how external resources are accessed, then define and document the public native-app API and its validation-report behavior.

Written by the indexing model from the issue text.