feat: add custom options

[douglaseggleton]

Allows you to add custom options to Embed that will be processed outside of Gr4vy.

<Gr4vyEmbed
  // Provide a list of custom options
  customOptions={[
    {
      label: 'Giftcard',
      method: 'giftcard',
      description: 'You will be asked to enter a giftcard',
      iconUrl: `data:image/svg+xml,...`,
    }
  ]}

  // Handle the submit for a custom option
  onCustomSubmit={({ method: 'giftcard' }) => {
    alert(`Paid by ${method}`);
  }}
/>

[gr4vy-code]

🚀 PR was released in v2.2.0 🚀