Store downloaded files elsewhere

[jw3]

Keeping downloads in the project dir is not ideal, but was nice for a quick turn on this. For a 0.1 we need to investigate a portable solution to this problem.

In Rust there is https://github.com/dirs-dev/dirs-rs which says

The library provides the location of these directories by leveraging the mechanisms defined by

• the XDG base directory and the XDG user directory specifications on Linux and Redox
• the Known Folder API on Windows
• the Standard Directories guidelines on macOS

[arosien]

I was looking at another extension and they use os.tmpdir to store the downloaded stuff.