Support file revisions

Hello PyDrive2 team,

First, thank you for developing and maintaining this package, I find it very helpful.

Second, I wanted to propose a new feature, which I personally find very useful (e.g. I am planning to use it for creating a simplified configuration management tool):

possibility to download a particular file revision by its id.

I have a code snipped that worked for me (based on GetContentFile function), but It has many shortcuts and potential flaws, therefore, I don't think it is good idea to create a PR with this code. Unfortunately, I don't have much experience working with the GDrive API to fix it myself.

@LoadAuth
def _GetContentRevision(
    file,
    rev_id,
    filename,
    chunksize=DEFAULT_CHUNK_SIZE,
):
    revisions = file.auth.service.revisions()
    file_id = file.metadata.get("id") or file.get("id")
    if not file_id:
        raise ValueError('No ID')

    def _download(fd, request):
        downloader = MediaIoBaseDownload(
            fd, file._WrapRequest(request), chunksize=chunksize
        )
        done = False
        while done is False:
            status, done = downloader.next_chunk()

    with open(filename, mode="w+b") as fd:
        rev = revisions.get(fileId=file_id, revisionId=rev_id)
        revision_metadata = rev.execute()
        rev.uri = revision_metadata["downloadUrl"]
        _download(fd, rev)

Hope you will find this feature useful as well.

-- Nikita

Comienza en pydrive2/files.py, en GetContentFile, y lee la referencia vinculada de la Google Drive revisions API; después, sigue cómo se envuelven los ID de archivo y las solicitudes de descarga. Se considera terminado cuando el paquete puede descargar una revisión de archivo especificada mediante su id, conservando el comportamiento de descarga existente.