Add support for older browsers

[ollyhayes]

There is another step before caballo-vivo can be used with IE support, we'll need to have another babel compiled directory:

• /lib-commonjs - as we have now
• /lib - run babel with ["env", { "modules": false }] - compile to es5 but keep es6 modules

there is another field in package.json - es2015, so we can have:

{
  "main": "./lib-commonjs",
  "module": "./lib-esm",
  "es2015": "./src"
}

The idea is that es2015 is untranpiled, and module is transpiled for our target - es5 for IE11.

This is a really good guide: http://2ality.com/2017/04/setting-up-multi-platform-packages.html#support-by-bundlers

[cristiano-belloni]

Closed by https://github.com/zambezi/caballo-vivo/pull/29