Allow creating arbitrary XML nodes in web.xml

Allow arbitrary XML in web.xml. Configured as follows:

```groovy
webxml {
			customXml = [
							[after:'servlet-mapping',
							tag:'resource-ref',
							xml: {
									'res-ref-name'('mail/KatalogMail')
									'res-type'('javax.mail.Session')
									'res-auth'('Container')
							}]
						]
```