Reopening #133

There shouldnt be much to say here. #133 was never solved and should simply be reopened. Since this is a breaking and from the app's perspective unsolvable bug for anyone using sass instead of scss extension it should not be closed again unless the sass format itself becomes deprecated.

To allow a fresh and uncluttered discussions I'll describe the bug anyways.

### Reproducing
* make addon using ember-cli-sass. Use default scss extension and add random addon.scss file to addon/styles
* create new app and install ember-cli-sass in it
* configure new app to use sass extension for ember-cli-sass plugin
* try to build new app

### Expected
app should be build containing styles as per documentation of the involved addons

### Actual
build fails with this very plugin trying to import addon.sass from the addon because of the sass configuration option being overwritten by the app
ember cli sass needs to maintain the plugins extension configuration and apply it there while it needs to use the apps configuration when building the styles of the app.

### Solution
Remove antipattern to import an "addon.whatever" file and require all plugin authors to require the developers to import all necessary styles explicitly. The whole concept is way to confusing anyways. If plugin authors want to provide a concise way of importing a huge list, they should make sets like bootstrap is doing it