Make CodePane support vertical scroll bars

[shreyasbharath]

First of all, let me say I love Spectacle!

I have switched over from PowerPoint to Spectacle and I am now using it to author a technical presentation with lots of code examples.

One of the issues I noticed with CodePane was rendering when the examples don't fit vertically on a slide. It doesn't seem to add a vertical scrollbar. However, it does add a horizontal scrollbar for examples that don't fit horizontally.

A vertical scroll bar would be pretty neat!

[kenwheeler]

Care to PR with a max height and overflow scroll?

[shreyasbharath]

Sure I can give it a shot. I am new to JS/NodeJS/React so please bear with me.