GET /
GET /(?P<user>[a-z0-9]+)
GET /(?P<user>[a-z0-9]+)/posts/(?P<post_id>[d+]+)
GET /(user)
GET /(user)/posts/(int:post_id)
GET /(user)/posts/(post_id:int)
GET /posts/(int:post_id)
POST /posts/(int:post_id)
GET /users/(int:user_id)/posts/(tag)