Add cache folder in user home dir

It will be used as a fallback if there is no internet connection. Otherwise get data from server and check if cache update is needed.

Other idea is to always hit the cache. If there is no hit then go fallback to internet. Problem is updating sheets. It can be solved by adding extra flag `--no-cache` and command `update` that will fetch all new sheets.

IMHO 1st option is much better.

The issue names no files, tests, or entry points. Start by locating the CLI code that fetches sheets and deciding where user-home cache behavior belongs; compare the two proposed cache strategies and define how offline fallback and cache updates should work before implementation. Done means the chosen behavior is specified and covered for online, offline, and cache-update cases.

Written by the indexing model from the issue text.