Be more DRY when referencing examples and excerpts. E.g.,
```jade
+makeExcerpt('quickstart/ts/app/app.component.ts', 'import',
'app/app.component.ts (import)')
```
can now be just
```jade
+makeExcerpt('app/app.component.ts', 'import')
```
Defined new mixin for examples named `makeProjExample` using this new
scheme.
The original `makeExample` has been left untouched.
Applied new mixins to quickstart.
closes#1420
5b1a84be4c