Use es6 modules to better support users that use tree-shaking bundlers

[frederickfogerty]

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• Please search through the existing issues (both open AND closed) to see if your feature has already been discussed. Github issue search can be used for this: https://github.com/imgix/drift/issues?utf8=%E2%9C%93&q=is%3Aissue
• Please take a moment to find out whether your idea fits with the scope and aims of the project

Is your feature request related to a problem? Please describe.
From #61

Can you use export in Drift.js? Mixing module formats causes issues when using webpack to convert modules. The old way didn't present an issue when we used Babel, but webpack appears to be more strict. We switched because webpack can "tree shake" unused modules.

Describe the solution you'd like
Use export default rather than module.exports. The problem with changing the usage right now is that it breaks the UMD build for direct browser sourcing (i.e. window.Drift). Another bundler will have to be used, such as Webpack, to support this.

Describe alternatives you've considered

• Browserify: this tool is currently used but doesn't seem to have a way to support export default in this way.

[fleck]

I'll try to put together an example of using rollup with babel to bundle. Webpack is great for building application bundles, but I like rollup better for libraries.

[frederickfogerty]

Hey there @fleck. Can you take a look at #70? I've used Webpack + Closure over there, and would love your feedback.

[frederickfogerty]

This was closed in #70