**Do you want to request a *feature* or report a *bug*?**
Feature

Right now babel plugin emits globals: https://github.com/facebook/react/issues/16604

```js
window.$RefreshReg$ = () => {};
window.$RefreshSig$ = () => type => type;
```

It would be nice to have them configurable. That would allow to use `import.meta` in environments like SystemJS and have simpler implementation:

```js
import runtime from 'react-refresh/runtime'
runtime.injectIntoGlobalHook(window)

System.constructor.prototype.createContext = function (url) {
  return {
    url,
    $RefreshSig$: runtime.createSignatureFunctionForTransform,
    $RefreshReg$: (type, id) => {
      id = url + ' ' + id
      runtime.register(type, id)
    }
  };
};
```

If you don't mind I could create PR with changes to react-refresh/babel next week.

environment:
```js
{
    "systemjs": "^6.1.4",
    "react": "^16.11.0",
    "react-dom": "^16.11.0",
    "react-refresh": "^0.6.0"
 }
```
);
    }
    return args;
  }
  let seenForRegistration = new WeakSet();
  let seenForSignature = new WeakSet();
  let seenForOutro = new WeakSet();
  let hookCalls = new WeakMap();
  const HookCallsVisitor = {
    CallExpression(path) {
      const node = path.node;
      const callee = node.callee;
      
      
      let name = null;
      switch (callee.type) {
        case 'Identifier':
          name = callee.name;
          break;
        case 'MemberExpression':
          name = callee.property.name;
          break;
      }
      if (name === null || !/^use[A-Z]/.test(name)) {
        return;
      }
      const fnScope = path.scope.getFunctionParent();
      if (fnScope === null) {
        return;
      }