**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 foundInside = findInnerComponents(
              innerName,
              firstArgPath,
              callback,
            );
            if (!foundInside) {
              return false;
            }
            
            
            callback(inferredName, node, path);
            return true;
          }
          default: {
            return false;
          }
        }
      }
      case 'VariableDeclarator': {
        const init = node.init;
        if (init === null) {
          return false;
        }
        const name = node.id.name;
        if (!isComponentishName(name)) {
          return false;
        }
        switch (init.type) {
          case 'ArrowFunctionExpression':
          case 'FunctionExpression':