add filter option

this pull request adds the ability to filter out files you do not want in the bundle.

Notice: the file will still be in the output, but it will go to `null`, instead of a destination...
this will be handy if a packer wants to use that in some way.

I think this will be useful, as a simpler way to solve this: https://github.com/substack/node-browserify/issues/356

and also this: https://github.com/substack/node-browserify/issues/387

and maybe this: https://github.com/substack/node-browserify/issues/314