support fs.readdir and fs.readdirSync

I think this is feasible, and I personally think this is really useful.

In node.js it's reasonably common for me to do things like:

``` javascript
// controllers/index.js
'use strict';
var fs = require('fs');
var path = require('path');

var ROOT_PATH = __dirname;

module.exports = function(path) {
var fnames = fs.readdirSync(ROOT_PATH);

var paths = fnames.map(function(fname) {
return path.join(ROOT_PATH, fname, '.js');
});

var modules = paths.map(function(path) {
return require(path);
});

return modules.map(function(module) {
// do something with the module
// i.e. if it's this is a mongoose schema bootstrapping utility, register the model.
// if this is a express controller bootstrapping utility, parse its methods and register
// matching routes.
// etc.
});
};
```

This allows us to abstract whole parts of the code, later on - even treat them as completely separate modules:

``` javascript
// app.js
var controllers = require('./controllers')

// bootstrap the controllers
controllers(/* something would come here such as an express app or a mongoose.Connection */);
```

If we could do this with browserify, things like bootstrapping an Ember app, could be automated. I know there are a couple of problems.

[Here's an analogous example that works](https://gist.github.com/yamadapc/9976164), though not exactly as one would hope it'd.

I know this has to do with limitations on the browserify module (which may be impossible to address) but this would be **really** cool.

Perhaps we could even parse things like:

``` javascript
['./a', './b'].map(function(name) {
return require(name);
});
```

Into:

``` javascript
[require('./a'), require('./b')];
```

Or provide an utility `requireMap` thing that resolved into that.

I don't know if this is a relevant issue. But I though this would be a nice thing, if it was possible.

At it's core, the main point was simply to add:

``` javascript
var fs = require('fs');
var files = fs.readdirSync('.');
```

To be transformed into:

``` javascript
var fs = require('fs');
var files = ['a.js', 'b.js', 'main.js' /* etc. */];
```

What do you think?