Add height, width options to slide numbers

[sangramjagtap]

https://github.com/gitbrent/PptxGenJS#slide-number-options

Default width assigned to slide numbers is insufficient for 3 digit slide number. (99+)
PFA screenshot above.

Would be great if we can add custom height,width as per requirement.

[gitbrent]

Thanks for the issue @sangramjagtap

I've increased the width of the numbers object. I'm not going to add w/h for it yet to keep it simple for clients.