Cache and revert

[alissa-huskey]

This patch provides a autoenv_cache function that can be used to cache variables in .env files when leaving is enabled. These are then reverted in autoenv_leave().

[hyperupcall]

Oooh, this looks useful ^w^, it seems to address some ideas in #19. Before I merge this I wanna get #205 merged, so tests can be added for this behavior

[hyperupcall]

While this is a useful feature for some people, I'm closing this because I think the addition would make the project more complex than what is necessary. There are other softwares that do better unloading and those can be used instead. This code is written to be simple and clear, so many features will be left-out.