Propose to allow code invocations inside XML syntax

Currently, the XML syntax only allows static values on attributes. But in many scenarios, the value can come from some variable or other code.

This PR extends the XML syntax to support special attributes (value prefixes with `@`) that include Java code.

```
<tone:header title="@title" />
```

Please note that the code isn't evaluated or validated in any way. The value is passed directly to the source output. Also note that I didn't implement an EL parser, so `obj.title` isn't transformed in `obj.getTitle()`.