Proposed solution to support primitive wrapper types in XML syntax

The first release of the `<tone:tag />` syntax only supports `String` for parameters. I have a few use cases where being able to use some `Boolean`s would be nice:

```
<tone:tag flag="true" />
```

(today this only works with `(@String flag)` and manually calling `Boolean.parseBoolean`)

This propose adds a conversion method to the builder that calls `Boolean.parseBoolean` for me.

One major drawback is that we loose static *value* checking (no way to know if you type a valid value). So we can get parsing exceptions on production (like `NumberFormatException`).

----

P.S. Please note that it isn't possible to generate the proper method invocation without the parsing call with panettone. To do that, the invocation point would need access to the declaring type, which isn't supported.