New attribute in <include> tag to extend hive

Here's variant attempt 2 to respond to the need described in issue #551. The syntax would be:

```
<include href="myfile.html" with="bar={{@foo}},baz=123"/>
```

Variable accessible from myfile.html are `$bar`, `$baz` (new vars) and `$foo` (existing vars).

See #618 for variant 1