Express Middleware Support

My initial thought after finishing up is that this functionality belongs in a plugin/component and not core because it consumes the API rather than adds to it. However, the marketeer in me also knows that `gluejs` won't survive the unsteady waters of node module development unless the n00bs can plug-n-play. With that said, this feature is pretty straight forward and a good ground floor to begin supporting `express` out of the box. 

I originally had a **highly** opinionated set of defaults that I eventually neglected for simplicity in setup. The interface I propose here only requires one parameter and all other options are left as opt-in – define them if you need them, ignore them and they'll be set to `glue`'s defaults. This gets you up on your feet quick.

Middleware can be defined with `app.use()`:
```javascript
var glue = require('gluejs').middleware;

app.use('/js/app.js', glue({
  include: './lib'
}));
```
Or at the route level:
```javascript
app.use(app.router);
app.get('/js/app.js', glue({
  include: './lib'
}));
```
The `basepath` defaults to the `include` path and `main` defaults to an `index.js` script in the `include` directory, so each of the above will default to:
```bash
$ gluejs \
     --include './lib' \
     --basepath './lib' \
     --main 'index.js'
```
And can be requested in HTML thusly:
```HTML
<script src="/js/app.js"></script>
<script> 
  console.log(window.Foo); 
</script>
```
The necessity for defining an include could be stripped out if we assume a convention of using `lib/` directory in the `cwd` for main include:
```javascript
app.use('/js/app.js', glue());
```
But this seemed a little opinionated and I wanted you to weigh in first so I added it as a separate commit. Let me know how this looks and if there is anything else you would like to consider.