New string formatting concept exercise

[BNAndras]

WIP but will need introduction, instructions, hints, etc. I'll also need to undo the formatting on the track config since it's pretty noisy.

[BethanyG]

I really apologize - got distracted, and this has sat too long. It might be best (between the docstring changes and the config.json changes) to re-submit this instead of trying to fix merge conflicts.

We've changed the docstring styles to Google - so you'll need to look at the other concept exercises to see how that looks.

I also noticed you don't have any tasks that cover <str>.format(). Weren't we going to add at least one or maybe two?