Update the database *after* printing

[metov]

The tldr client frequently wants to update pages, which requires a network request, which takes a few seconds. This seems to me like not ideal UX for a quick reference tool. When I'm trying to look up some basic syntax of a command, giving an answer quickly is more important than giving a definitive one - if I want the definitive, I'd be looking at the man page or searching on the internet instead of using tldr.

From what I can tell, the current flow is:

1. User requests page of topic
2. tldr checks cache, maybe updates the page
3. tldr shows the page

I think a better UX would be:

1. User requests page of topic
2. If page is present in the cache:
   1. tldr shows it
   2. tldr updates the page (if cache is stale)
   3. If the new version is actually different, tldr prints a message: "This page has been changed from the one you just saw. Run tldr again to see the new version."
3. If page is not present in the cache at all:
   1. tldr prints a message: "Hold on, fetching page from the database..."
   2. tldr updates the page
   3. tldr shows it

This is of course more complex, but I think justified by the benefit to the user. What do you think? I'm willing to author a PR if the idea sounds reasonable.

[metov]

I think an alternative, less invasive solution would be for me to simply create a systemd timer that runs tldr --update_cache every day or so, which would ensure that the cache is never stale.

If this is the recommended solution, though, it would be useful to have a new --update-all argument, which pulls all of the pages rather than what's in the cache. The entire tldr pages repo is currently only 27 MB, so the latency-space tradeoff seems reasonable.