**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"
 }
```
id: sigCallID,
            init: t.callExpression(t.identifier('$RefreshSig$'), []),
          });
          
          
          if (path.node.body.type !== 'BlockStatement') {
            path.node.body = t.blockStatement([
              t.returnStatement(path.node.body),
            ]);
          }
          path
            .get('body')
            .unshiftContainer(
              'body',
              t.expressionStatement(t.callExpression(sigCallID, [])),
            );
          
          
          if (path.parent.type === 'VariableDeclarator') {
            let insertAfterPath = null;
            path.find(p => {
              if (p.parentPath.isBlock()) {
                insertAfterPath = p;
                return true;
              }
            });
            if (insertAfterPath === null) {
              return;
            }