Investigate alternative nesting API

The current API for showing a view looks like:

``` js
this.showChildView('main', MainView, {force: show});
```

There are two problems with this. The first is the third argument contains both options to pass to `show` (like `force`), but also the view options. I could solve this by having 4 arguments, but...yuck. The other problem with this is that the developer never instantiates the view, which conceals the fact that we're instantiating it with additional options. That additional option is the `el` property, which is pretty important!

Perhaps it would be better make the API more like:

``` js
this.showChildView(new NewMainView({
el: this.region('main')
}), {force:show})
```

This approach solves both problems above without any loss of functionality. Now the third argument is _just_ options for show, and the code clearly shows that the view is being created with an element. It's also just much nicer to create the view yourself, I think.

The `childViews` option would be unchanged, however. That currently looks like:

``` js
var RootView = Puppets.View.extend({
childViews: {
main: {
view: MainView,
options: {color: 'blue'}
},
'.modal': ModalView
}
});
```

Start by locating the existing showChildView and childViews entry points described in the issue and compare how they receive view and show options. Done means reaching a settled decision on the proposed instantiated-view API while preserving the stated childViews behavior; the issue does not name files or tests.

Written by the indexing model from the issue text.