Add support for a pre response hook for plugins

[m1h43l]

We need to be able to hook into the response before it is send over the network.

Sometimes the response needs to be modified before sending f. e. CORS support, #34 . CORS needs to add some extra headers to the response.

[m1h43l]

@NielsLiisberg As the response struct is already available to the plugin as a parameter can we already add the HTTP header on the existing pre request hook? Then at least for setting the CORS headers there is no need for any additional hooks.

[m1h43l]

Reponse data structure is also available in the PRE_REQUEST hook. So we can also modify the response there. Is good enough for me atm.