Add support for NewsBlur

Quirks:

* NewsBlur doesn't include the title when querying a single feed, so we cache
  it locally
* Credentials are stored via cookies. Cookies are stored in curl handles which
  are recreated for every request. The user has to specify a "cookie-cache".

I has certainly needs more error checking when accessing the JSON structure, and it should probably make use of the json-c refcounting. I'll add those if/when the general structure is acceptable.