Add attribute @value to a pipeline:parameter

Instead of a element `<pipeline:value>` inside `<pipeline:parameter/>` it would be nice to just use a attribute `@value`

It would look like 

```
<pipeline:transformer name="someStep" xsl-path="someStep.xsl">
      <pipeline:parameter name="someParameterName" type="xs:string" value="someParameterValue"/>
 </pipeline:transformer>
```