Add `scrollToElement` option #5

Useful if you want to capture a part of the screen that contain a certain element.

The value can either be a CSS selector string or an object like:

{
    element: '#foo',
    offsetFrom: 'top', // Accepts `top`, `right, `bottom`, `left
    offset: 10 // Pixels
}

The above object is not set in stone. Happy to receive feedback on it.

I'm also not sure how the object value should be implemented for the command-line interface.