Added onBeforeShow callback

[hgascoigne]

If I have somehow missed this in the documentation, please feel free to give me a hard time. :)

If not, I added an onBeforeShow callback. This is particularly useful if I need to prepare parts of the interface before showing a step. For example, I needed the ability to open an off-canvas drawer before showing the steps within that drawer so the steps would be properly positioned.

As the name implies, this callback will fire before onShow and before any set delay begins. I also wrote a simple test to make sure it fires.

It was useful for me so I thought I'd create a pull request and see if it was also useful for anyone else.