Honor OS defaults for default cache location

[isotes]

• rename to import.pw relative to OS default cache location
• honor $XDG_CACHE_HOME and $LOCALAPPDATA and fall back to "$HOME/.cache"

Note: this fixes the "$HOME folder bloat" mentioned in Issue #36 and follows OS conventions on Linux and Windows as mentioned in https://github.com/importpw/import/issues/36#issuecomment-660370606. For macOS it falls back to $HOME/.cache since handling macOS would require uname wrangling which I would prefer to avoid, mostly for #38. On my macOS installation $HOME/.cache already exists since it is used by other tools as well so it is a compromise I personally could live with.

Additionally, it renames the directory to import.pw to be easier to google.

Please let me know if you would prefer a different approach.

[TooTallNate]

This looks good to me, though ultimately I'd like to get the macOS paths correct as well. Like I mentioned in #38, I think this logic belongs in the import script itself, rather than outside of the script (and out of our control) in a snippet, so I don't mind the additional complexity.

[isotes]

Ok, I will update the PR with support for macOS

[TooTallNate]

Great, thank you!