[Feature]: Cache docset feeds

[Morpheus636]

Describe the Feature

Make it so the docset feeds are cached instead of downloading and deleting each time.

• This would also add a command zeal-cli cache update which would delete the existing cache and download the most recent one from the reomte repository.
• A --update-cache option should be added to the zeal-cli install command to use the existing behavior instead of the new behavior.
• If the cache does not exist, the program should automatically download it.
• If the cache is old, it should warn the user and tell them how to run zeal-cli cache update

Closing Requirements

• Remove the code which downloads and deletes the feeds for each command
• Refer the install command to a cache directory
• Create a cache update command
• If the cache directory doesn't exist or is empty, run the cache-update function
• Create a file which stores the date and time that the cache was last updated (within the cache dir)
• If the cache was last updated more than a week ago, prompt the user to update the cache.
• Write tests for the above