NPM package isn't published in es5

Nobody has claimed this yet.

While trying to use Martini in a deck.gl terrain layer, I discovered the NPM package isn't transpiled to es5 (pure JS developers would run into syntax issues).

Could you publish an es5 version as the default along side es6 distributable?

You might find [this article](https://www.grzegorowski.com/publishing-npm-package-with-rollup-babel-and) useful, it seems most folks are using babel (and this [plugin](https://github.com/rollup/rollup-plugin-babel)) to do the transpiling.
Here's a snippet of their rollup config.
```
babel({
// ignore node_modules/ in transpilation process
exclude: 'node_modules/**',
// ignore .babelrc (if defined) and use options defined here
babelrc: false,
// use recommended babel-preset-env without es modules enabled
// and with possibility to set custom targets e.g. { node: '8' }
presets: [['env', { modules: false, targets }]],
// solve a problem with spread operator transpilation https://github.com/rollup/rollup/issues/281
plugins: ['babel-plugin-transform-object-rest-spread'],
// removes comments from output
comments: false,
}),
```

Start by inspecting the package's Rollup publishing configuration and the existing ES6 distributable. Run the package build and verify that the default NPM artifact is transpiled to ES5 while the ES6 distributable remains available.

Written by the indexing model from the issue text.