Allow for easier access to pretrained models

[BramVanroy]

It would be great if the access to pre-trained models is simplified. Downloading large files manually from Google Drive on a remote server is very cumbersome. Ideally, a built-in downloader could be provided that allows you to do something like:

sent2vec download wiki_bigrams

Although, I would also be happy with a service other than Google Drive that allows for easier downloading of the pretrained models.