GitHub folder download&zip

Github web ui allows downloading and zipping a whole repository but it does not allow doing that inside subfolders.

This plugin extends the github webpage UI so that it is possible to download and zip a subfolder.
It works by looking for all the links in that subfolder, fetching raw files from that links and compressing them into a single zip file.

Because all this can be done in the context of the logged in user, this can work seamlessly without needing to set up any authorization keys.

Known limitations:
- binary files are currently incorrectly zipped, because text() is read from fetch response
- only files from the top files are added to the zip (the files from subfolders are not)