Proxy configuration

Add a "proxy" configuration key for setting the proxy on both SoapClient and libxml for fetching the WSDL. The proxy can be specified either as a string ("host:port") or as an array containing 'host', 'port', and optionally 'login' and 'password' keys.

Honestly, I could not test whether the proxy authentication with login and passwort really works because I don't have a proxy that requires authentication. Without authentication the proxy settings work well for me.

Initially I developed these changes against the 3.x branch but since it got merged to master I rebased on master and created a new pull request.
Once we have a valid proxy configuration we need to merge it back into `soapClientOptions` or update code generation accordingly as [the generated client will not have the proxy configuration included](https://github.com/wsdl2phpgenerator/wsdl2phpgenerator/blob/master/src/Service.php#L109) otherwise.