iron-image demo

### A plain `iron-image`.

### `sizing="cover"` expands the image to cover all of its specified size.

### `sizing="contain"` expands the image to fit within its specified size.

### Use the `--iron-image-width` property to set the width of the image wrapped by the `iron-image`.

### Use the `--iron-image-height` property to set the height of the image wrapped by the `iron-image`.

### No placeholder is shown by default.

### The `preload` attribute shows a placeholder element in front of the image before it has loaded. Use the `--iron-image-placeholder` CSS mixin to style it.

### Use the `placeholder` attribute to specify a background image for the placeholder element.