Add prettier auto-formatting

[starwed]

Prettier is a library for auto-formatting javascript code. I'd tried it in the past, but it used to mangle the formatting on certain files in the repo.

Seems like these bugs are now fixed, so

• Add prettier as a dependency
• Set up commit hooks that auto-format javascript files in the repo
• Run the script on all existing files
• Remove any jshint ignore line directives in favor of more explicitly scoped disabling. (Prettier was assuming it was ok to move the comments around, which didn't work for ignore line.)

I'm fine with all the default settings prettier uses, so I didn't add any special configs.

Fixes #951