Add options.include and options.excludeOverrides.

Making it easier to customize what files make it to the output tree.

Despite [my recent SASS dependency efforts](https://github.com/g13013/broccoli-compass/issues/27#issuecomment-57319512) (which are starting to work decently), it also was helpful to me to have a few more ways to customize the tree output. For example, I needed to make sure that partials (`**/_*.{scss,sass}`) were copied into the output, because I needed later filters/writers to be able to detect if they were changed (like to invalidate of Jade templates via `include`ed/`extend`ed dependencies).

And I figure I'd throw in `options.include` while I was there too.