Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
Try this:
```
var cleanURI, input = {};
do {
  var result = ps.prompt(null, Zotero.getString('pane.items.attach.link.uri.title'),
    Zotero.getString('pane.items.attach.link.uri'), input, null, {});
  if(!result || !input.value.trim()) {
    // User cancelled
    return;
  }

  cleanURI = Zotero.Attachments.cleanAttachmentURI(input.value);
} while(!cleanURI);

Zotero.Attachments.linkFromURL(cleanURI, itemID);