Add an optional progress bar to Sequence

[jdpigeon]

I thought it would be nice for the library to have an easy-to-use progress bar like jspsych has. I went ahead and made same changes to the Sequence in class in order to achieve that in this PR.

I'm not sure if adding this directly to Sequence is this best idea as this will break if the user tries to nest progress bars. Maybe it could be added to the global Lab.js 'engine'?

Also, I was pretty shocked to put this whole thing together and then find some CSS styles for a progress bar! I didn't see anywhere in the library where these would be applied, so I'm curious what the story is.

Cheers, and let me know what I can do to tidy this up.

[FelixHenninger]

Oh, apologies for the force-push! 😬