Use when you need to track a method call w/ the `trackEvent` utility.
The value returned will be passed as `data` to `trackEvent`.
If a function is used for the `trackingFn`, it must return an object w/ a `name`, and optional `data`
@param  {Function|String} trackingFn Either a function to build up the event data or a string event name.
@param  {Object} [options] An opject of options
@param  {Boolean} [options.sendReturnValue] Whether or not to send the return value as data
@return {Function} The decorator function
@example <caption>String Tracking</caption>

class Foo {
@track("Button Clicked")
buttonClicked() {
// ...
return { some: "data" };
}
}
@example <caption>Function</caption>

import FooTracker from "./foo.tracker";

class Foo {
@track(FooTracker.clicked)
buttonClicked() {
// ...
return { some: "data" };
}
}

// foo.tracker.js

export default function clicked(data) {
// data = { some: "data" }
data.otherData = "other data";

return {
name: "Button Clicked",
data
};
}
@example <caption>String Tracking w/ sendReturnValue false</caption>

class Foo {
@track("Button Clicked").sendReturnValue(false)
buttonClicked() {
// ...
return { some: "data" };
}
}
function track(trackingFn, options={ sendReturnValue: true }) {
  let decorator = function(target, name, descriptor) {
    const hasDescriptor = typeof descriptor.value !== "undefined";
    const fn = hasDescriptor ? descriptor.value : target;

    function trackDecorator() {
      let fnReturnValue = fn.apply(this, arguments),
          value = null;
      
      if (options.sendReturnValue) {
        value = fnReturnValue;
      }

      trackEvent(typeof trackingFn === "string" ? 
        Object.assign({ name: trackingFn, data: value }, options) : 
        trackingFn.apply(this, [value]));

      return fnReturnValue;
    };

    if (hasDescriptor) {
      descriptor.value = trackDecorator;
    }
    else {
      target = trackDecorator;
    }
  };

  decorator.sendReturnValue = function(value = true) {
    options.sendReturnValue = !!value;
    return this;
  };

  return decorator;
}