Minify RDMD production builds.

[rafegoldberg]

I set up TerserWebpackPlugin to run on our builds a while back, but for some reason the minified output would break the ReadMe app whenever I imported it!

https://github.com/readmeio/api-explorer/blob/67ebc1dde38871bee0c8127166ae3dd90668051d/packages/markdown/webpack.config.js#L18-L27

Didn't quite determine what was causing this issue, but we should get back to it ASAP so we can reenable minification.

[rafegoldberg]

(cc: @dok — of note wrt/our discussion re: bundle size!)