Added a "Key Step" feature for toggling view of a key step from anywhere using 'K' key.

I added the concept of a "Key Step" that can easily be referenced from anywhere in the presentation by pressing the "k" button on your keyboard.

Often during presentations you have a slide that provides an overview the topic or contains relevant information that you want to refer back to often. Every presenter knows the frustration of having a key slide that causes people to keep asking you, "Hey, can you go back to that slide that showed ____". This feature allows you to set that "Key" step up very simply just by adding a "keyStep" class to that step's div element. 

After that, you can press the "K" key from anywhere in the presentation to toggle your view and go to (or from, if you are already viewing the key step) the key step from wherever you currently are. This allows you to move forwards or backwards through the deck to view that step and then go back to where you were.