Move templatess to `templates/$action/$engine.tmpl`.

This gives us engine-specific default templates, currently including Postgres
and SQLite templates. The addition of the `--template` option to the `add`
command allows the user to specify a template of any name, which will make
customization much easier.

Still to do:

* Update the docs with the new template names.
* Update the installer to install the templates in the right place.
* Update the installer to move existing templates to subdirectories,
  and emit a message about user- or project-specific custom templates
  needing to be moved, as well.

Ref issue #50.
Add oracle and mysql templates.

The oracle templates are non-transactional.
Document the new --template option and locations.