allow execFile() options to be configured via wintersmith config.json

one issue we have run into as we are building a very large project is the size of compiled SASS assets blowing up the default `maxBuffer` option for the plugin's `execFile()`.

wintersmith (or node, rather) ends up barking this:

```
  200 /USCRSF/USCRSGBL/js/gui_library.js BrowserifyPlugin 1028ms
  500 /USCRSF/goodyear/css/goodyear.css SassPlugin 2294ms

  error USCRSF/goodyear/css/goodyear.css: stdout maxBuffer exceeded.

  200 /USCRSF/USCRSGBL/css/vendor/images/ajax-loader.gif StaticFile 1ms
```

so, i added the ability to pass `execFile()` options from the `config.json` under  `sass.exec_opts`. our implementation looks like:

```json
{
  "plugins": [
    "./node_modules/wintersmith-sass", 
    "./node_modules/wintersmith-browserify",
    "./plugins/static",
    "./plugins/dataserver"
  ],
  "sass": {
    "exec_opts": {
      "maxBuffer": 614400
    }
  }
}
``` 

i changed the plugin registration to one line using a better pattern as well as breaking the `execFile` callback out to stay within an 80 character limit (not sure if this was an intentional convention on your part or not, but the plugin was under 80 cols, so i kept it there).