Added ability to have wildcards for custom_headers

Firefox will fail to load webfonts if they don't have a CORS header and they're coming from Rackspace CDN.  This pull request adds the ability to have config like this:

```
  custom_headers:
    "\.(ttf|otf|eot)$":
      "Access-Control-Allow-Origin":"*"
```

And then when syncing assets, Rackspace CDN will then send back down the correct header.

Committed with a passing spec.

Comments or OK to merge?