Add skip_download option to download_folder to return the list of files without download

[wkentaro]

Close #310
Close #309

Why?

In #309, @o-laurent explained an interesting use case where he wants to select which file to download in a Google Drive folder based on its file name.

What is this?

If you pass skip_download=True to download_folder, you can get list of files as list of tuple of (id, path, local_path).
After that you can just use gdown.download to download the ones you want.

# Example:
files = gdown.download_folder(..., skip_download=True)
for file in files:
    if file.path.endswith(".txt"):
        gdown.download(id=file.id, output=file.local_path)