#871 Add options to control the border size and color

[Jens Riisom Schultz]

I want to be able to specify a distance of (f.e.) 42 pixels from the border of the window, to the actual text.

It would be nice if you could customize the color and alpha of this border, as well.

As an example of this, see the Commodore 64's terminal: It has a huge colored border...

(Yes, there are other feature request regarding the border, but they are too specific, and not as dynamic as I need)