Add --disallow-only option

Using `describe.only(...)`, `test.only(...)`, etc. can be useful in your local test environment, but you almost never want that filter checked in to source control. This patch adds a `--disallow-only` flag which will create a test failure if that syntax is used. The intended use would be to set the flag for builds on your CI server, but not in your local development environment (except for possibly a pre-commit hook).