Possibility to specify a border of type "rectangle" and an orientation "landscape/portrait"

[pokyah]

Hi @koenderks,

I'm still playing around with rcityviews and what I'm missing is the possibility to draw a rectangle border.

Also it would be nice if we can produce a ggplot object with a final specific ratio that would respect an aspect_ratio argument.

Like this we can produce plots that will perfectly fit in standard paper aspect ratios.

What do you think of this?

I could work on it and submit a PR like we did previously for the geocoding stuff.