RSS feed fields can now be defined in the query files

Previously, some of the RSS feed fields were specific to CFPB.  Now they can be defined as needed in the _queries/<type>.json file like so:

```
"feed": {
    "feed_title": "CFPB Blog",
    "entry_url": "$$permalink$$",
    "entry_title": "$$title$$",
    "entry_content": "$$content$$",
    "entry_author": "$$author$$",
    "entry_updated": "$$date$$",
    "entry_content_type": "$$html$$"
  }
```

The '$$' strings should describe the attribute name on the individual item for each field.  So if your item's author is stored as item.author, you'd use $$author$$.