add schema validator for config.json

[edeustace]

This adds some simple validation to pack-question and serve-question.

• checks that the config.json is present
• checks that dependencies.json is present
• checks that index.html is present
• validates config.json with 1 base schema that checks for pies: [ { pie: { name: version: }}] and then validates the individual pies with the schema located in $pie/docs/schema/config.json.

The command will fail if any of the above checks fail.

original notes

When we run pack-question or serve-question we should first check that:

1. the files are where we expect to find them
2. the config.json has the correct structure
3. the dependencies.json has the correct structure
4. the html file is present (check for instance declarations too?)