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?