feat(windowId): Added ability to specify a windowId for modal window

So after updating from 0.7 to 0.9, I notice I can no longer specify an ID for my modal windows (as I don't control that part anymore).
I see that you can only specify custom windowClasses and think it would be good to also allow a custom windowId as they're known to be much more performant.

I'm using overlays quite a bit throughout my app as they sit on top of the page, giving the user the feeling that they never go anywhere, such as:
 - login
 - register
 - overview pages, etc

so my SCSS structure looks somewhat like:

```files
scss/app.scss
scss/core/_base.scss
scss/pages/_home.scss
scss/overlays/_login.scss
scss/overlays/_register.scss
scss/overlays/_game-overview.scss
scss/overlays/_property-overview.scss
```

_login.scss:
```CSS
#loginModal {
   width: 800px;
   // Some other stuff 
}
```

_game-overview.scss:
```CSS
#gameOverviewModal {
    width: 900px;
    // Some other stuff
}
```

So anyway, when you're working with a bunch of designers, they'll make sure to make every part of the application look very special (like a nice login page with lots of little frills and an overview page with more frilly stuff) and would like to specify that as an ID, as though it was it's own page.

I don't want to do .loginModal as there's only going to be one, and I don't want the CSS to have to search my DOM for that class to apply the styles.

Good idea?