Expose Jest transform function to help with Jest-Babel-Webpack compatibility

[davidtheclark]

There are a few annoying things about running Jest within codebases that use Webpack generally, Batfish specifically.

• Babel config is created behind the scenes for Webpack, combining Batfish defaults with user options (babelPlugins, babelPresets). Jest doesn't know about this; so if you want to write Jest tests for files that rely on this Batfish-generated Babel config, you have to reproduce it in a .babelrc file or something else Jest can read.
• Batfish allows Webpack to allow you to import non-JS files. Jest doesn't run Webpack so it will break on non-JS imports. You need to configure Jest to ignore those.

To deal with this, I think Batfish should export a jest-transform function that can be passed to Jest's transform config option, and it will try to take care of whatever transforms Jest needs to work with Webpack things.

cf. jestjs/jest#1468 (comment), https://github.com/atecarlos/webpack-babel-jest/blob/master/index.js