performance

There is an easy way to get much better performance. Currently, everything in bower_components/vendor and app are _considered_ when determining if a rebuild is needed or not.

Now this means stuff "magically" works, but this means as an app and vendor grow. Copious time is spent merely figuring out if some input file changed or not.

We have several options, likely worth exploring. Most affective would be:
1. only watch `app/styles` by default
2. give users an API to add additional "input paths"

An example API could be:

``` js
var app = new EmberApp();

app.cssPaths.concat([
app.name + '/styles/**/*.scss',
'bower_components/material-design-lite-src/src/**/*.scss'
]);
```

---

Another thing, would be to add matcher, to only both detecting specific file types. This will require some fixes to our change detection, but again may yield a good win.

Unfortunately, I am unclear if such a matcher exists. Would it be possible for us to only watch files with the following extensions? `.css` `.scss` `.sass` ?

---

Some recent micro-optimizations that yield a nice boost: (reducing costs of stats + crawls by 2x), but ^ would be a substantial win in many cases.

part 1: https://github.com/aexmachina/broccoli-sass-source-maps/pull/5
part 2: https://github.com/ember-cli/broccoli-caching-writer/pull/58