Add Example with Custom Label for Addon Prices

[douglaslise]

Add example using data-recurly="addon_price" for plans with tiered add-ons:

buffer = $.map(plan.plan.addons, function(addon) {
  // usage add-ons need to be displayed separately
  if (addon.add_on_type === 'usage') return;

  const label = `<label for="addon-${addon.code}">
                  ${addon.name}
                  (<span data-recurly="currency_symbol"></span>
                  <span
                    data-recurly="addon_price"
                    data-recurly-subscription="${plan.subscription.id}"
                    data-recurly-addon="${addon.code}"
                  />)
                </label>`;
  const input = `<input type="text"
                  data-recurly="addon"
                  data-recurly-addon="${addon.code}"
                  data-recurly-subscription="${plan.subscription.id}"
                  id="addon-${addon.code}"
                  value="0"
                >`;

  return label + input;
}).join('');

This relies on this Recurly.js PR:

• Fix Tiered Unit Pricing Labels recurly-js#790