Support multiple entry-points

[runspired]

When multiple final bundles will be produced, it can be advantageous to keep a single tree for as long as possible.

Allowing for multiple entry-points can enable us to avoid transpiling the same file if it were to occur in more than one tree. Making broccoli-dependency-funnel aware of this concept is better than the alternative (fork => dep funnel => merge) as it enables caching already walked subgraphs instead of needing to parse and walk N times for N trees.

If this is something we want, it likely needs to happen at the mr-dep-walk level with the entry-point configuration passed into mr-dep-walk from broccoli-dependency-funnel.

cc @stefanpenner @twokul

[stefanpenner]

sounds good