/* @flow-runtime */ turns on assert and annotate even if they're disabled in plugin config

This is a:

- [ ] Bug Report
- [ ] Feature Request
- [ ] Question
- [x] Other

Which concerns:

- [ ] flow-runtime
- [x] babel-plugin-flow-runtime
- [ ] flow-runtime-validators
- [ ] flow-runtime-mobx
- [ ] flow-config-parser
- [ ] The documentation website

---

### What is the current behaviour?
If the `.babelrc` contains `["flow-runtime", {"assert": false, "annotate": false}]`, a file with a `/* @flow-runtime */` annotation will override this and turn on assertions and annotations.
---

### What is the expected behaviour?
`/* @flow-runtime */` just enables `babel-plugin-flow-runtime` if it is not already enabled (with my new `optInOnly` PR).

Without reading the docs it's not at all obvious that `/* @flow-runtime */` turns on assertions and annotations; by itself it looks like an opt-in just like `/* @flow */`. Why not just require assertions and annotations to be explicitly turned back on if they're disabled in the plugin options?