Support absolute positioning for all elements on title page

[mojavelinux]

As an alternative to margin_top, which shifts the element down from the current location in the flow, support absolute an position for each element on the title page using the top or bottom property.

The top property should position the element relative to the absolute top of the page (ignoring the margin). The bottom property should position the element relative to the absolute bottom of the page. The top and bottom properties are mutually exclusive.

[mojavelinux]

We could also consider left and right properties.

[mojavelinux]

As I stated in #66 (comment), we've decided to focus on a programmatic solution to this need. Refer to that comment for the reasoning.

You can find an example in the docs for how to create an extended converter to customize the title page.