schemer-cli

Tom suggested a simple CLI. I agree this would be useful for trivial projects where migrations are serializable as SQL.

- [ ] schemer-cli crate
- [ ] optional adapter trait for creating an adapter instance from command line args
- [ ] optional adapter trait for creating migrations from files in directories
- [ ] this could be based on a simple metadata TOML, or even just filenames like `[id].[one or more dependency ids].up.sql` for Postgres adapter
- [ ] inclusion of each adapter would be a feature on the schemer-cli crate

Would make standalone use as trivial as
```sh
cargo install schemer-cli --features postgres
schemer -a postgres:'localhost/mydb' up [directory of migrations]
```

No files or tests are named. Start by reviewing the existing crate structure and the proposed schemer-cli entry point, then map the checklist for adapter arguments, migration files, metadata or filenames, and feature-gated adapters; done means the documented cargo install and schemer command work for the Postgres example.

Written by the indexing model from the issue text.