specify time intervals for custom agenda integration

[wuqui]

I do something like a weekly review and I rely on the agenda clocking information for days in agenda view. If I'm looking back at yesterday or the week before all the TODOs I had only saved in journal files have disappeared.

I know this is done to avoid cluttering and slowing down org-agenda by adding too many files and I really like it! In fact I would probably use org-journal even if it was only for this single feature -- thanks a lot for that! However, could you add an option to let users customize the agenda integration by specifying the time intervals they need to have included? For example, many might want to still have the last week included for the purpose described above and I guess 7 more files wouldn't slow down the agenda all that much?