Add titleStyle as prop in CodeSlide

[asantos00]

This adds the titleStyle prop so we are able to customize the title of the code slide

[Arnotjevleesch]

As we can see here : https://github.com/Arnotjevleesch/spectacle-code-slide-vs-renderer/blob/master/presentation.pdf

The fixed position :

spectacle-code-slide/src/CodeSlideTitle.js

Line 4 in 978dffe

position: 'fixed',

makes title visible on every slide before code slide for the export.

A relative position in a customize title style would fix the issue.