Extensible sourcemap configuration

Before this change, `app.options.sourcemaps` was just a list of file extensions. This changes it to be an object, so that it has room for future source-map related options, and so that we can offer a simple `enabled` flag.

My intent is that this will be a stable API that other addons can use. Any preprocessor that can generate sourcemaps should respect `app.options.sourcemaps.enabled` and `app.options.sourcemaps.extensions`.

broccoli-sourcemap-concat, broccoli-uglify-sourcemap, and ember-browserify have already been updated to use this convention.
Should we just call this `sourceMapConfig`(and other references to this name) since we're just changing it to that later in the options?