Basedir and run options

There's two improvements in this pull request:

1) it allows the browserify `basedir` option to be passed through the middleware.

We have an application where the browserify-middleware is in one package and the things to be browserified are in multiple other packages. With this fix browserify uses the correct base path.

2) The `run` option allows to intersperse bundles with files to be added via `bundle.add` together with files that are bundled with `bundle.require`.

This option is helpful/we need because we have a global bundle with just exports and then page-specific bundles, which have modules inside by themselves but also a kickstart file that should be started with the bundle. Otherwise it would require another external file just for the kickstart.

Any thoughts on these? Any chance of getting them included?
Browserify requires `basedir` to be a `string` or `undefined`