Missing Babel dependencies in 5.0

[goto-bus-stop]

There are a few issues with #69 and the 5.0 release. The babel config uses relative preset/plugin names like "env", which causes babel to resolve it relative to the project's directory, not the directory where extract-loader is installed. If the user's project does not use Babel 6, or uses a different version, that is a problem.

The fix for that is to do require('babel-preset-env') in the babel config, so it resolves to the version of that package installed with extract-loader.

I tried to fix that but ran into a different issue. babel-preset-env is not listed as a dependency, but as a devDependency for extract-loader. That is an easy fix of course, but I'm not sure if it's the best thing to do to depend on babel-preset-env. If the goal is only to support ES modules, would it be a better idea to use babel-plugin-modules-commonjs?