Add disk backed cache - and find/fix other startup delays.

[Josverl]

total startup time is some 7.8 secons=ds just s=to show 'stubber --help'

reading the current versions takes approx 1.8 - 2 seconds each startup - that could be cached still leaves 6 other seconds unaccounted for is this introduces with rich- / rich-click ?   https://pypi.org/project/toolcache/

remove cache entries once they reach a specific age

@toolcache.cache('disk', ttl='48 hours') def f(a, b, c): return a * b * c