Optional context configuation added to jetty

[zamaterian]

Its now possible to add a context to the jetty adapter;
enable jetty adapter to mimic the same context as when deploying a war file, then the filename is used as the context name.

ex : customer.war in tomcat -> context is /customer ex: http://localhost:8080/customer

[weavejester]

Thanks for the pull request, but my plan is to handle this case with a wrap-context middleware function, rather than changing anything in the Jetty adapter.