Use XDG_CACHE_HOME as download path

[vbrandl]

This fixes the problems described in #27

• no more problems with non-writable download directories (except if a non-writable XDG_CACHE_HOME is configured, which makes no sense)
• it is possible to run multiple versions of the downloader/checker without path conflicts
• the downloader 'detects' if it has been updated but the downloaded checker is an older version
• the OS/user knows what files are not required for the downloader to work and can be removed safely (when running again, it will be rewritten to the cache, but that's how caches work)

On my system with the default XDG settings (XDG_CACHE_HOME=$HOME/.cache), the checker is stored as $HOME/.cache/editorconfig-checker/2.0.3/bin/ec-linux-amd64

Running XDG_CACHE_HOME=$HOME/.cache2 cargo run stores the binary in $HOME/.cache2/editorconfig-checker/2.0.3/bin/ec-linux-amd64, as expected.

Actually all the other editorconfig-checker.* projects might (should?) use this pattern, too (one might discuss the details about how the path should be structured) so different downloaders can share the already downloaded checker.

Closes #27