Prettier enhancements: feedback/state & options

I think it would be useful to have some discussion about the prettier functionality on runpkg. Some points to start with:

1. Button press feedback
2. Undo option
3. Basic config options

## 1: button press feedback and state

Currently it can be hard to tell that any formatting has taken place. Although this function (`prettier.format`) isn't async, I think some state handling (including start & finish state) in the UI would be helpful. In my mind, possible states to alert the user to are:

- start format
- finish/successful format
- finished format with no change (`prettier.format(fileData.code, opts) === fileData.code`)
- error
- format reverted (see point number 2)

These states could be signified with text, icons and/or toast messages. Preferences? I would lean towards (accessibly) icons possibly enhanced with toast messages.

## 2: undo format

I think it would be useful to be able to revert the code formatting without having to refresh the page. I'm happy to implement it in a PR. Any objections? Any opinions on how to represent this has successfully run in the context of point number 1 (above)?

## 3: basic config

This is a trickier and more subjective feature IMO: one one hand, prettier is supposed to be opinionated, but - on the other - it does provide certain config options (quotes, semi...) for a reason. Do we want to allow users to select basic config options? E.g. currently we use the default options which include `singleQuote: false` which means that something like `lodash-es/add` only changes on format due to single -> double quotes rather than any major formatting issues.