feat(templating) add ability to compose templates that are defined in the same html that composes them

In reference this associated [stackoverflow question](http://stackoverflow.com/questions/39834025/is-it-necessary-to-define-a-template-in-a-separate-file)

I would like to propose that aurelia have the ability to compose templates that are defined in the same html that composes them. This would be in contrast to the current restriction (correct me if I'm wrong) that composed templates must be defined in their own separate html file.

The proposed html could look like this, though I note there are plenty of other potential ways to do this:

```

${a-property-on-containers-binding-context}

```

No files, tests, or entry points are named. Start by examining the existing template composition behavior and the referenced Stack Overflow question, then determine how inline named templates should be represented and composed. Done means the proposed inline-template example works and its behavior is covered by tests.

Written by the indexing model from the issue text.