Add brick to get computed style of element

[twschiller]

Context:

• As a user I need to determine if an element matches a certain style, but the CSS class is randomly generated (via CSS module)
• Browsers have a method to get the style attributes of an element: https://developer.mozilla.org/en-US/docs/Web/API/Window/getComputedStyle

Implementation Sketch:

• Add a Get Element Style brick
• Brick should be root aware
• Take an optional selector for an element