Recursive download: add export

[serebrov]

The docs mentions a limitation of the recursive download:

Downloading directories
Downloading a directory is done by the same command as downloading a file, but with the --recursive flag:

gdrive files download --recursive <file id (directory)>
this will create a directory with the same name as the Google Drive directory and copy all files recursively. It may take a while before you get any output, but you should get a status on which directories and files are created and downloaded.

Note: any Google Docs files will be silently ignored when doing this.

This PR updates the recursive download to automatically export docs to docx, spreadsheets to xlsx, and presentations to pptx.

With this addition, files download --recursive can be used as a full backup tool to save Google Drive as local files.

The PR also changes the process from "walk the tree with no output for a long time, then download" to "walk the tree and download at the same time" (this is done in a separate commit, so can easily be removed, although I think it is more useful this way)..