Add dry-run option?

[rod-glover]

Performing a dry run was recently suggested. Alembic doesn't provide this ability directly.

Alembic does, of course, perform migration actions inside a transaction; if any error occurs during the migration, the transaction is rolled back. However, that is not a dry run.

This SO answer shows how to do it; basically enclose the dry run in a transaction and always roll it back. This will at least show Alembic's output of what would happen on a successful real migration. We should look into what level of detail (verbosity, debug output) Alembic can provide and turn that on appropriately for dry runs. The default level of Alembic output is terse.