Expose original 'done' function

[modax]

Hello,

I need to call browser-sync in 'done' callback and I would like to do this in addition to the original 'done' callback which performs quite a lot useful stuff. However, current API leaves me no other choice but to replace original done callback without a way to do a "super" call. It would be great if original done function (starting at https://github.com/shama/gulp-webpack/blob/master/index.js#L30) would either be publicly exposed or passed to the overriding 'done' as third parameter.

[shama]

I'd accept a patch that exposed it as a separate method.

[modax]

Sure. See #61