Changes to tooltip buttons

Support for alternative text for 'skip' button on last step, and 'disabling' of prev/next buttons on first/last step

I think this is what issue #74 is referring to. I think it makes more sense to have disabled states for prev/next, than to remove either, or change the wording of 'next'. But 'skip' definitely seems ripe for change. I also think it might be nice to change 'Next' to *not* exit, and for Enter to hit 'skip'/'done' on the last slide - maybe the default button should be highlighted visually ...