Feed cursor pagination (#49)

* Expose promoted tweets in feed output

* Add cursor-based feed pagination output

SCHEMA.md

@@ -8,6 +8,8 @@
 ok: true
 schema_version: "1"
 data: ...
+pagination:
+  nextCursor: "optional-cursor"
 ```
 
 ## Error
@@ -25,6 +27,7 @@ error:
 - `--yaml` and `--json` both use this envelope
 - non-TTY stdout defaults to YAML
 - tweet and user lists are returned under `data`
+- timeline-style list commands may also return `pagination.nextCursor`
 - `article` returns a single tweet object under `data`
 - `status` returns `data.authenticated` plus `data.user`
 - `whoami` returns `data.user`