Add `<tour-proxy-anchor>` component

[phrei]

In discussion #159 I had the following problem:

I'm using an external component (in this case a calendar component) that contains a button group to switch views. I'm not able to set the tourAnchor-Directive on this button group (as the code is not in my domain).

My first solution was to create a div and position it absolutely over this button group and use the tourAnchor-Directive on this new div. If the layout changed somehow and these buttons were not in the same place anymore or changed in dimension, the absolutely positioned div was wrong (especially visible, if backdrop is enabled).

So instead of positioning this "artificial" tourAnchor-DIV I've created a directive which receives a CSS selector. This directive finds the element I want to overlay, gets it's bounding rect and sets the div's position and dimensions accordingly.

In this branch you can see a demo of the directive. As a test I try to highlight the label of the button instead of the whole button (to showcase an element, that is not in my domain but in angular material's).

Additionally theres an Input for padding (which uses the technique from your docs to set padding and remove margin).