Add a `--since` and `--untill` flags to the timeline command

[BackEndTea]

Currently the basecamp timeline command has the following options to fetch only a set of data from the timeline.

--all              Fetch all events (no limit)
-n, --limit int    Maximum number of events to fetch (0 = default 100)
--page int         Fetch a single page (use --all for everything)

However sometime I want to answer the question of "What happened last thursday?".

Currently i can do the following:

• Fetch the whole timeline and filter
• Take a guess at the amount of events i need
• Paginate back until i find the correct date.

By allowing --since and --until flags with a date time attached to it, i could more easily determine what happened in a specific time range. Omitting one of the since or until flags would fetch everything after the since or before the until.