Support different basedir per browserify() call

I have an application structure where the express server is being instantiated in a different module from the handlers and the handlers are injected. Basically something like this:

```
+--app
    \
     +--http-server
     |  \
     |   +--express 
     +--handler
        \
         +--dependencies
```

Because the module search is relative to ```__dirname``` of the caller (which in fact is the module that creates the express-app) browserify-middleware isn't able to pull in the browser dependencies from the other branch.

A "basedir"-option on the browserify() call would solve that problem, e.g.:
```
app.use("/js/base.js", browserify(["dependency1", "dependency2"], {
    basedir : __dirname
  });
```