Feature request: position  & css var's for fill color

It would be great, if we could do something like this:

```js
QRCode({
    msg: 'xxxxxxxxxxxxx',
    dim: 120,
    pal: ['var(--primary-color)', 'var(--secondary-color)'],
    x: 123,
    y: 45
})
```

that will result in:

```svg
<svg x="123" y="45" viewBox="0 0 37 37" width="120" height="120" fill="var(--primary-color)" ...
```