Allow downloading a single post with --url

[JessiAuro]

Right now, if you pass a url of a single post to PatreonDownloader (such as /posts/<post-slug> or /posts/<post-id>), it downloads the entire Patreon, which makes it very difficult to debug issues with a specific post in a large Patreon.

The application should support processing a single post to simplify hunting down bugs related to specific posts.