Option to download at a specified rate (to avoid exceeding query rate)

Feature request: Option to download at a specified rate (to avoid exceeding query rate)

Description: Currently, IG has a rate limit. For users with a large number of posts (>2000), it's almost guaranteed to hit the `Query rate exceeded` limit before you can download all the posts. One way around this is to set a limit to the query rate from the client-side, so that we don't hit the limit. We can then leave instalooter running in Terminal overnight, and all posts will be downloaded.

Acceptance Criteria:
- new Files Option `-r NUM` or `--rate-limit NUM`, which specifies the number of files per minute to attempt downloading.
- complete downloading all media from profiles with large number of posts (>2000) without hitting `Query rate exceeded` limit by setting a very low NUM.