fix #488 - support user field for completion

- a few other backends use `user` instead of `username` or `login`
  (e.g. gopass-jsonapi, keepass2csv importer, fpm2 importer)
- shorter to type when extracting field separately (e.g. CLI/clipboard) tools

pass/Controllers/PasswordDetailTableViewController.swift

@@ -250,6 +250,9 @@ class PasswordDetailTableViewController: UITableViewController, UIGestureRecogni
         if let username = password.username {
             section.item.append(Constants.USERNAME_KEYWORD => username)
         }
+        if let user = password.username {
+            section.item.append(Constants.USER_KEYWORD => user)
+        }
         if let login = password.login {
             section.item.append(Constants.LOGIN_KEYWORD => login)
         }