New inapp config options and long-TLD cross-subdomain support

• new Mixpanel config option inapp_link_new_window controls whether links in inapp notifications will open in a new window/tab (defaults to false)
• new Mixpanel config option inapp_protocol takes precedence over built-in protocol-agnostic resource links (for instance loading https://example.com/image.png rather than //example.com/image.png)
• cross-subdomain tracking now works for TLDs with more than 6 characters, like .company

Example usage of new configuration options:

mixpanel.init('<MY-TOKEN>', {
  inapp_link_new_window: true,
  inapp_protocol: 'https://',
});