I've updated the PR with a new config format.

The previous package.json "mangle" field is still supported for compatibility purposes.

Now, in either mangle.json or package.json, both can specify a "minify" field containing configuration for Terser:

{
  // "minify" for Terser (https://github.com/terser-js/terser#minify-options-structure)
  // Note: works as a key in both package.json or mangle.json
  "minify": {
    // Legacy/convenient way to set mangle.properties.{regex,reserved}:
    "properties": "^_",
    "reserved": ["_dom", "_constructor"],

    // Optionally specify Terser compress configuration.
    // Supported values: https://github.com/terser-js/terser#compress-options
    "compress": {
      "hoist_vars": true,
      "reduce_funcs": false
    },

    // Optionally specify Terser mangle configuration
    // Supported values: https://github.com/terser-js/terser#mangle-options
    "mangle": {}
  }
}

@marvinhagemeister for now I've left out a couple top-level configuration items like module and ecma because Microbundle is going to automate those soon and we don't want folks to tweak them thinking it's necessary.

@hassanbazzi most of the Terser options folks would want to change are on mangle and compress, so for those that's what this does - sets some defaults then allows arbitrary overrides.

For Terser's root options, we really can't allow arbitrary customization. Doing so would break output
in the very near future, when Microbundle begins outputting ES2017 alongside the current downlevelled ES5 (see #304). When we do this, we'll be running Terser in ES5 mode for the 3 classic script bundles, and {ecma:8,module:true} for modern bundles. Since ecma and module are the two main root options we're not exposing here, I think that with the modern bundling setup in place there won't really be a reason to customize any root options. We can always add them manually once we have a better idea, but for now I don't want folks to try customizing ecma only to intentionally break it in 1.0.