**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"
 }
```
const {key, customHooks} = signature;
    let forceReset = hasForceResetComment(scope.path);
    let customHooksInScope = [];
    customHooks.forEach(callee => {
      
      let bindingName;
      switch (callee.type) {
        case 'MemberExpression':
          if (callee.object.type === 'Identifier') {
            bindingName = callee.object.name;
          }
          break;
        case 'Identifier':
          bindingName = callee.name;
          break;
      }
      if (scope.hasBinding(bindingName)) {
        customHooksInScope.push(callee);
      } else {