Code formatting and optional cleanup script

[SaFrMo]

This app being View Source-ready and self-contained is a very useful feature, so like @everestpipkin and @wheany referenced in #25, a required build step feels like something to stay away from.

An optional cleanup command seems like one good way to get the more consistent style that @caelinsutch suggested - that way folks who use NPM can adhere to styling rules and PRs from folks who don't can be quickly standardized after merge.

To that end, I tried out adding a cleanup process that looked like this on this PR:

• Add a package.json file
• Add prettier as a dev dependency
• Add a cleanup script that runs prettier on all HTML, CSS, and JS files
• Add rules to standardize the current styling (single quotes, semicolons, 4-space tabs)

This is working smoothly on my end - npm run cleanup is totally optional but the code is much more consistent when it's run. Let me know what y'all think of this - I know it's adding some extra complexity and am happy to talk through other options too!

[everestpipkin]

This feels like a nice, optional way to approach this, which won't get in the way of editing or compiling but that will let contributors clean up the code periodically. I'm going to merge it in.