feat: update settings move under updates, with an interval

updates.autoCheck (was updateCheck) and updates.updateSources (was
releases) sit together with updates.autoCheckInterval, a duration (24h
by default) for how often lazysecrets looks for a newer release on its
own.

README.md

@@ -102,7 +102,7 @@ names and metadata of what you've loaded in a cache file readable only by
 you, to show it at once next time (`cache: false` turns it off). It talks
 only to the services you use it with, and collects no usage data. The one
 call of its own is a daily check for a new release, on GitHub or else
-git.kebcloud.no, which `updateCheck: false` turns off.
+git.kebcloud.no, which `updates.autoCheck: false` turns off.
 
 ## Repositories