Some problems with pragmas

This is a:

- [X] Bug Report

Which concerns:

- [X] babel-plugin-flow-runtime

---

### What is the current behaviour?

1. When warnings are enabled via the options, `@flow-runtime assert` does nothing; `shouldWarn` still is true in the `ConversionContext`, and only warnings are emitted.

2. The `@flow-runtime` annotation is not recognized when it is followed by a `"use strict"` directive, because babel attaches the comment to the directive; e.g.
```
// @flow-runtime assert
"use strict";

console.log((1: string));
```
does not assert.

---

### What is the expected behaviour?

1. The pragma should override the options completely.
2. The pragma should still work even in the presence of a strictness directive.

---

### Which package versions are you using?

0.11.1, also reproducible on https://codemix.github.io/flow-runtime/#/try