JS config never gets garbage collected.

I discovered this issue while trying to catch a memory leak in our build tool. The way cosmiconfig loads '.js' config prevents it from ever getting garbage collected, even if userland code no longer needs. `require-from-string` package [creates a new `Module` record](https://github.com/floatdrop/require-from-string/blob/master/index.js#L24) which then permanently stored in memory. You can see it with this sample:



```js

const cosmiconfig = require('cosmiconfig');



const loadRC = require.resolve('cosmiconfig/lib/loadRc.js');



function loadOnce() {

    const explorer = cosmiconfig('example');

    return explorer.load(__dirname)

        .then(() => console.log(require.cache[loadRC].children.length))

}



let count = 1000;

function loop() {

    if (count > 0) {

        count--;

        loadOnce().then(loop);

    }

}



loop();

```



Provided there is a non-empty `example.config.js` nearby, you should see that `require.cache[loadRC].children.length` grows by 1 after every load and never gets cleaned up.

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