Allow to pass configuration function to express()

The configure() function is quite handy, especially in CoffeeScript with `@` being used to access properties on this.

I find myself repeatedly creating an app and running configure() on it w/o an environment, just so I'll be in the app context and could use the `@` sugar, than running the rest of the code inside that function, e.g.:

```coffee
app = express()
app.configure ->
  @use express.static __dirname
  @set '....'
  # ...
```

What if `express()` simply allowed to give it a function, and immediately execute it in the context of the app?

```coffee
express ->
  @use express.static __dirname
  @set '....'
   # ...
```