Reconsider cache folder

Reconsider caching in the library's install folder. The GitHub issue tracker is rife with confusion about the permission warning (#9), or outright uncaught exceptions (#209). Finally do something about it. 😄

Some example approaches:

* System temporary dir
* These aren't for caching though. These can be evicted by the OS whenever.
* XDG standard cache directory
* Brought to my attention [in this comment](https://github.com/john-kurkowski/tldextract/issues/196#issuecomment-668974052).

Review the library's current cache handling and the permission warning in #9 and exception in #209; the issue does not name files or tests. Compare the proposed temporary-directory and XDG cache approaches, then confirm the chosen cache location and that permission failures no longer produce the reported warning or exception.

Written by the indexing model from the issue text.