Add extractEnvelope option to client

I'm working with a service that sends some data before and after the actual SOAP envelope that causes the expat parser to fail. Here is an example:

```
--MIMEBoundary_2af39b87d6c9f2ba6d1c13d73db88d0bfdf9414786fcc315
Content-Type: application/xop+xml; charset=UTF-8; type="text/xml"
Content-Transfer-Encoding: binary
Content-ID: <0.3af39b87d6c9f2ba6d1c13d73db88d0bfdf9414786fcc315@apache.org>

<?xml version="1.0" encoding="UTF-8"?>
<soapenv:Envelope 
. . .
</soapenv:Envelope>
--MIMEBoundary_2af39b87d6c9f2ba6d1c13d73db88d0bfdf9414786fcc315--
```

I added an option that extracts the SOAP Envelope from the received data, and passes that on to the parser.