A way to enable flow (in strict mode) for all files by default

I want `@flow strict` to be the default in my project. In some cases, for particular files, I want to downgrade to non-strict `@flow` or to disable Flow altogether.

Is there any way to do this, without having to type `// @flow strict` in every single file?

- When I set `all=true`, I get thousands of errors from inside `node_modules`, and I can't see how to change this without breaking other things.
- Even with `all=true`, I can't see a way to make `strict` the default.