plugin-instructions: add complete button?

[ChristopheBossens]

For an experiment we extended the instruction plugin the following way:

• It includes a complete button between the previous and next button. When the complete button is used, the next button becomes disabled on the final page and only the complete button will finish the trial.

In our case, this allowed us to present different stimuli across multiple pages, and have the participant select a stimulus by using the complete button. Another possible benefit is that users would not need to go back to the final instruction screen when they want to review a specific instruction from the start of the sequence.

If you think this is a useful feature to have in general I can create a proper implementation.