Added content zoom modifiers.

[scmanjarrez]

I couldn't find a way to change font size dynamically, but I found these options to increase the content size that can palliate the problem #125

[antonycourtney]

This is a great change, thank you!
I'll probably try and change the "Zoom-in" modifier to Cmd-+ instead of Cmd-= because I think that's more standard, but otherwise looks and works great.