Fixes bug introduced when we moved `Token` into the func's parameters,
out of PostParams.

Moves the ID and token params out of PostParams and into the func's
parameters. A currently-unused `collection` parameter is left in for the
future, when the backend supports updating and deleting collection posts
via slug, not post ID.

Previously, they all said the same thing

writeas.github.io/docs -> developer.write.as/docs/api

Plus break up internal request building so we can optionally send query
parameters with some requests (like DELETEs!)

Plus change Post's ModifyToken to Token.

Plus fix Client post func and rename SubmittedPost to PostParams.

Includes ability to make authenticated requests and get posts.