XJB customization

Add the ability to configure xew from xjb like this one:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<jaxb:bindings xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
               xmlns:xjc="http://java.sun.com/xml/ns/jaxb/xjc"
               xmlns:xs="http://www.w3.org/2001/XMLSchema"
               xmlns:xew="http://github.com/jaxb-xew-plugin"
               version="2.0">
    <jaxb:bindings>
        <jaxb:bindings schemaLocation="test.xsd">
            <xew:xew collection="java.util.HashSet" collectionInterface="java.util.Set"/>
            <jaxb:schemaBindings>
                <jaxb:package name="com.test"/>
            </jaxb:schemaBindings>
        </jaxb:bindings>
    </jaxb:bindings>
</jaxb:bindings>
```
Maybe not perfect but improve customization following XSD, Class, field ...