DO NOT MERGE: Adds a command for files in an npm package

[futuremint]

Fixes issue #1146

Additional description

Please review the output of running "npm run dist" in this PR. It adds an additional temp directory called .tmp-npm. I intend to publish the npm package from that folder. In its current state this PR uses Babel to down-compile what is in .tmp-es to general ES6. The idea is that we don't want to force any particular babel presets onto our users, but that we can be fairly sure they're using some kind of JS compiler in their toolchain downstream from this package to target different JS versions (such as ES5 or whatever they want).

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Reviewer checklist

• While the contributor's branch is checked out, run npm run dist and compare the generated JS files to the source JSX files.

[interactivellama]

I think this is looking good. We can always go more transpiled. We can't go less though in the future. The "@salesforce" namespace needs to be added to the package, so devs don't have to change that in the future. I'm going to test with create-react-app today.

[interactivellama]

The update on this is that we are hoping to do multiple versions in one tag:

• a lightly transpiled ES6 that maintains modules in /components/
• a CJS version that means if you use import { Button } from 'design-system-react', you won't need Babel transpiling of files in node_modules which will make this format compatible with a non-ejected create-react-app. CJS can be swapped out for "modular ES5" in this instance, I think.

[interactivellama]

Closing in favor of #1210