Enable full datastore_search api for datastore/dump endpoint

It's pretty handy to filter tabular data and download it as a CSV so that [casual users can open it as a spreadsheet](http://sunlightfoundation.com/blog/2014/03/11/making-json-as-simple-as-a-spreadsheet/). It looks like the [dump endpoint](https://github.com/ckan/ckan/blob/master/ckanext/datastore/controller.py#L22) might be able to support this functionality by passing along a few more query params to datastore_search.

Holler if this would be useful and I can submit a PR. Cheers.

```
data_dict = {
'resource_id': resource_id,
'limit': request.GET.get('limit', 100000),
'offset': request.GET.get('offset', 0),
# example:
'filters': request.GET.get('filters', {})
}
```

The dump endpoint is in ckanext/datastore/controller.py around line 22; start by comparing its query handling with datastore_search. Determine which datastore_search parameters should be passed through, including filters, limit, and offset, and verify that filtered requests produce the intended CSV output.

Written by the indexing model from the issue text.