Unknown keys with valid scan codes are not handled.

I am working on a german keyboard and want to capture key events of the german umlaut ü.
The following lines of code prevent this:

glutin_window/src/lib.rs

Line 301 in c9b89f3

virtual_keycode: Some(key), scancode, ..

glutin_window/src/lib.rs

Line 319 in c9b89f3

virtual_keycode: Some(key), scancode, ..

Here, in both cases, a valid virtual key code is required for the event to be fired (Some(key)). If this requirement is omitted (tested localy), an input event for the piston library can be thrown (of course with an unknown key, but valid scancode). This allows the user to handle previous unhandable key inputs.

If a change in that direction is desired, I can provide a pull request implementig this feature.