Provide query plan in JSON format

Today there is only a text representation of the query plan, but it would be useful to have a JSON version for portability and machine consumption.

To control whether EXPLAIN should produce a text or JSON output we could augment the EXPLAIN syntax or we could introduce a query option. It's worth discussing which one makes more sense.

To avoid maintaining two code paths for explain (TEXT and JSON), I recommend that internally our code should always generate the JSON plan, and then have a function that can convert the JSON plan to the conventional textual representation.