Add xml request body parsing + xpath

[hhalex]

Request body should be accessible when crafting the response.
Multiple possibilities can be studied:

Request body

<rootnode>
  <value>
    hello
  </value>
</rootnode>

api.yml

rules:
  - matches: POST /test
     response: !Inline [200, "<ok>{{body.xml.rootnode.value}}</ok>", "application/xml"] 

Or with a handlebars helper
api.yml

rules:
  - matches: POST /test
     response: !Inline [200, "<ok>{{#xpath body //value}}<anothervalue>{{@this}}</anothervalue>{{/xpath}}</ok>", "application/xml"]