Although we'll now end up with only `mustache.js` in the git repo,
being written in ESM syntax, the npm package will still be as before:

- `mustache.js`: UMD, meaning CommonJS, AMD or global scope
- `mustache.mjs`: ESM

By making the main source code an ES module placed in `mustache.mjs`,
what used to be in `mustache.js` is now built based off of the ES module
source inside `mustache.mjs`.

This is done primarily to avoid breaking existing projects already using
mustache.js, by keeping the UMD-version in `mustache.js` part of the git
repository in addition to the minified version in `mustache.min.js`.

After experiment with several compilers;

- Babel
- TypeScript
- Rollup

and examining their build output, [Rollup](https://rollupjs.org/) was chosen
because of the UMD output it creates which closely resembles what we've
historically had in the previous `mustache.js` source code.

The contents of `.js | .min.js` files has been generated by running
the following npm script:

```
$ npm run build
```

Also change linting w/eslint to target the `.mjs` file, since the output
of rollup that ends up in the old `mustache.js` file is not sensible to
do linting on.

http://greenkeeper.io/

Also minor .eslintrc change to be able to use `process.exit()`,
instead of having to `throw new Error()` to exit the process.