Shell script migrations

It might be interesting for tern to have migrations that are simply shell scripts that can do arbitrary tasks. tern could act as a proxy to the PostgreSQL server and expose a pseudo connection via the `PG*` or `DATABASE_URL` environment variables. Then a migration could have any logic written in any language (even just using `psql` could be really valuable).

No files, tests, or entry points are named in the issue. Start by reviewing tern's migration execution flow and how it connects to PostgreSQL, then determine the scope for running shell-script migrations and exposing PG* or DATABASE_URL. Done would require an agreed design, implementation, and tests for arbitrary migration commands.

Written by the indexing model from the issue text.