Add xml support for specifying body and matching rules separately

[caoilte]

With the JSON DSL it is possible to specify the body and matching rules separately. This is useful for scenarios where the JSON contains a lot of boiler plate that never changes and there are similar scenarios for XML where having to specify the XML using the DSL is cumbersome.

If adding support to specify the body and matching rules separately is agreeable I'd be happy to give it a go.

[uglyog]

PR are always welcome