Rework to act on documents rather then configs

This pull request uses Browserify through the API and allows acting on documents rather than having it in your docpad configuration file.

``` javascript
---
browserify:
	require:
		'./myfile.js':
			expose: 'file'
	ignore: ['ignorethisfile.js']
---
var myfile = require('file');
var ignored = require('./ignorethisfile');
```

This fixes both #2 and #1. @balupton, mind testing out with your stuff? Thanks!