Preserving bpack.hasExports on reset()

I went digging to figure out what was causing substack/node-browserify#887 and found that the `hasExports` setting for `browser-pack` is externally managed by browserify under various conditions, including the `b.require()` method.

Using watchify, you call `.bundle()` again on `update` which internally calls `.reset()` to recreate the pipeline, which creates a new `this._bpack` instance.

Because `hasExports` is not set in `this._options`, the `hasExternal` setting never makes it back into the new `_bpack` instance, meaning subsequent bundles behave differently.

I've fixed it by remembering the setting immediately before the pipeline is recreated, and immediately afterwards reapplying the previous value. 

I considered adding it to the `opts` provided to `this._createPipeline` but didn't want to introduce any side effects, the implementation I went with is more consistent with the way `this._bpack.hasExports` is set in all the other cases, so it will now have exactly the same behaviour initially and after any pipeline reset.