New configuration option: `submodules` (+ submodules are now disabled by default)

[asottile]

To improve performance -- and because most of the time you're not linting the submodules themselves in pre-commit.ci -- submodule checkouts are now opt-in.

You can opt into git submodule checkout by specifying `submodules: true` in the `ci` section of your `.pre-commit-config.yaml`.

For more information: https://pre-commit.ci/#configuration-submodules

Anthony