Cache plugins on user cache dir

[josevalim]

Given the plugins are immutable, we can download and cache them somewhere global. So the idea is:

1. Download them and write them to disk to the cache directory (if not yet cached)
2. Then read the zip files (without cooked mode)

For the cache directory, we can use this (copied from bumblebee):

  defp adbc_cache_dir() do
    if dir = System.get_env("ADBC_CACHE_DIR") do
      Path.expand(dir)
    else
      :filename.basedir(:user_cache, "adbc")
    end
  end