Pass all config in constructor

Is there a plan to pass config as following?
```
var Metalsmith = require('metalsmith');
Metalsmith({
"source": "src",
"destination": "build",
"plugins": [
{"metalsmith-drafts": true},
{"metalsmith-markdown": true},
{"metalsmith-permalinks": "posts/:title"},
{"metalsmith-templates": "handlebars"}
]
})
.build()
```

Start by reading the Metalsmith constructor and the current .build() entry point, then trace how source, destination, and plugins are configured today. Compare that API with the issue's example and define what behavior must be supported before validating the public interface with the project's existing checks.

Written by the indexing model from the issue text.