Target browsers in Babel config for CJS/ESM builds

[wKovacs64]

"browsers": [
  "> 1%",
  "last 2 versions"
],

This will be required if #8 is implemented, as the current CommonJS/ES module builds contain features that don't work across all the browsers this project aims to support (template literals, in particular).

It should really be added regardless, as people may even be wanting to circumvent the browser field already and explicitly import a non-UMD build on their own. Currently, when attempting to bundle a non-UMD build with webpack, the included UglifyJsPlugin will error during minification as it can't handle ES2015 features yet. (There are workarounds for this, such as bringing in the next version of UglifyJs or using a different minifier all together.)