title: checking a safari browser for the ability to install an extension
body: <p>I'd like a web page to offer a browser extension to Safari users. This web page already offers extensions to Google Chrome and Firefox users.</p>
  
  <p>In Chrome, the Javascript expression <code>typeof chrome.webstore.install !== 'undefined'</code> comes back true if it's possible to offer the user the Chrome extension.</p>
  
  <p>In Firefox, it's  <code>typeof InstallTrigger !== 'undefined'</code>.</p>
  
  <p>Is there a similar expression that will return true if it's possible to install a Safari extension?  </p>