Recognize npf as valid param for /posts/* endpoints

[nostalgebraist]

This PR modifies TumblrRestClient methods that retrieve posts, so that they accept npf as a valid parameter name.

This allows the user to retrieve posts in NPF, which is currently impossible using public methods of TumblrRestClient.

Thus, it will allow users of pytumblr to obey the following recommendation in the API docs:

Eventually, all posts will be NPF, so we strongly encourage leveraging NPF JSON via npf=true for post content when consuming posts via the API.

(For context, see discussion at tumblr/docs#36.)