Add `toString()` method alias for `url`

[Jan Buschtöns]

The code shown in #271 would be nicer, if Link had a toString() method, that returns the url.

Then you could do:

<Ui::Button
-  @link={{link "signup" query=(hash callback=(get (link "checkout") "url"))}}
+  @link={{link "signup" query=(hash callback=(link "checkout"))}}
>
  Please sign up first to continue
</Ui::Button>