Implement very basic async readFile for tests

This is for usage of .env with somethingToAutodetect. This API is not
actually supported in the browser, but we need to make it work for our
own tests.
Use cssstyle 0.2.20 instead of cssstyle-browserify

Also un-pin the dependency-pinning of cssstyle introduced in eaef6729c4f2c4a03fad65ae8b49fc74841923f6, now that we know 0.2.20 doesn't suffer from that bug.
Version 1.0.3
Revert "Use cssstyle 0.2.20 instead of cssstyle-browserify"

This reverts commit 07fa4f8636e355ee8b261c47bd714c63d18356e4. Closes #912; closes #913.
Version 1.0.4