public void preview(final ConfigSource configSource, final String format) {
        try {
            previewInternal(configSource, format);
        } catch (IOException ex) {
            throw new RuntimeException(ex);
        }
    }
Runs the preview subcommand.

It receives a ConfigSource and a String as parameters to be called from Ruby (embulk/runner.rb).
embulk-core/src/main/java/org/embulk/EmbulkRunner.java