Development: add a pause mechanism with user feedback when hitting rate limits

[aaribaud]

Problem

When fetching a user's toots, the high fetch rate is bound to cause the Mastodon instance to rate-limit, which in effect causes all requests to fail with a message indicating the date and time until which rate-limiting will be lifted.
Release v0.2.0 of mastotool ignores these rate-limit responses, and therefore continues trying to fetch:

1. without indicating to the user that a rate limit has been hit, and
2. causing the instance to keep rate-limiting ad infinitum

Simple solution

1. Define two running states for the fetch phase: fetching and rate-limited.
2. Initial state is fetching.
3. In fetching state, upon receipt of a rate-limit response, record the end-of-limit date and time and switch to rate-limited state.
4. In rate-limited state, upon reaching the end-of-limit date and time, switch back to fetching state.
5. In fetching state, behavior, including user feedback, remains unchanged with respect to v0.2.0.
6. In rate-limited state, no API request is performed, and the user feedback shows the time remaining until rate limit is lifted.

Notes

This may involve modifying the go-mastodon library if it does not provide enough info to the user code about the rate-limiting responses yet.

References

Conversion at https://mastodon.art/web/statuses/104737809996624450