[marian] Automate Tatoeba-Challenge conversion
This allows conversion of marian models from the Tatoeba-Challenge repo through the command line,
with instructions at `scripts/tatoeba/README.md`. This was previously impossible.
Tests are in `examples/` because the conversion requires examples dependencies (wget and pandas).

In my opinion, it would be a lot of work for very little benefit to remove these dependencies.

The goal of this PR is to allow @jorgtied to upload his own models.