if the button is disabled then don't perform its click action if it isn't an input

If you use a non-input element for the next/previous buttons (e.g. a link) you can get into a state where multiple steps are shown on the screen at once but only one is functional.

To prevent this from happening I've added some code so that the next/previous actions are only run if the links are enabled.

This is a cleaned up version of pull request #26.