Add pre-trained vector model to GitHub

[davidmezzetti]

Currently, the pre-trained vector model is stored on Kaggle. Put a copy of this file on the next GitHub release. This will allow automation/docker builds.

# Can optionally use pre-trained vectors
# https://www.kaggle.com/davidmezzetti/cord19-fasttext-vectors#cord19-300d.magnitude
# Default location: ~/.cord19/vectors/cord19-300d.magnitude