Package config

This PR adds ability to configure some es6ify options (at this point two) via the `package.json`:

```json
{
  "browserify": {
    "transform": [ "es6ify" ]
  },
  "es6ify": {
    "filePattern": ".js.es6$",
    "includeRuntime": true
  }
}
```

Just wanted to get your lgtm @domenic before I merge this in.

We should also discuss here what additional options we should make configurable via the `package.json` and how to merge it with transform options passed in directly (if we ever go that route [as indicated in this branch](https://github.com/thlorenz/es6ify/tree/v2-api)).

We also should think about deprecating configuration options done via JavaScript as they will become redundant and are not a good way of doing this anyhow.