Script to download a document as plain text

[schoettl]

I need to manage a shared plain-text file and Google Docs seems to be a good choice to edit the file. It only lacks a scriptable plain-text downloader, so I forked this repo.

I also updated some of your code:

• The new file extension for text documents seems to be .gddoc. The JSON content of the files seem to be compatible, the script still works.
• When I share a file from Google Drive, the first share link displayed is different from the link the script expects. So I added this URL format.