Template block with-metadata argument

If you supply a 2nd positional argument, then the data will
be passed to the template as:

```
{
  ...attributes,
  data = data
}
```

For example

```
[template, partial$foo.json, with-data, from=100, to=200]
---
{{#each data}}
The data item from foo.json is {{.}}.
The global metadata is:
    from: {{@root.from}}
    to:   {{@root.to}})
{{/each}}
---
```

Author: osfameron

lib/template-block.js

@@ -33,7 +33,7 @@ function register (registry, config) {
         const self = this
 
         self.named('template')
-        self.positionalAttributes('dataResourceId')
+        self.positionalAttributes(['dataResourceId', 'withMetadata'])
         self.onContext('open')
 
         self.process((parent, reader, attributes) => process(parent, reader, attributes, config, self));
@@ -44,13 +44,18 @@ function register (registry, config) {
 function process (parent, reader, attributes, config, block) {
     try {
         // resolve json or .yml data
-        const data = getResource(attributes.dataResourceId, config)
+        var data = getResource(attributes.dataResourceId, config)
 
         // compile .hbs template
         setupHandlebars(attributes, config)
         const source = reader.getLines().join('\n')
 
         const template = handlebars.compile(source)
+
+        if (attributes.withMetadata) {
+          data = { ...attributes, data }
+        }
+
         // output
         const output = template(data)
         const parsed = asciidoctor.load(output)