Bundle map/reduce

Hey @aroben.

I wanted to make sure it'd be :cake: add processors that generated metadata on every asset and then aggregated it during the bundle process.

The process should be

1. Define a pre/post processor to generate metadata per file. Thats pretty simple now we have processor metadata.
2. Define a bundle processor to aggregate many asset metadata pieces into a single value. This part is less clear right now. We have an exposed "required_asset_hashes" set of assets, but its considered internal and deprecated. So I think that needs to be formalized somehow.
`:required_asset_hashes` isn't a good api I want to be exposing.

https://github.com/sstephenson/sprockets/blob/fdba7fbfddbbab3c7498714c346ae2d9d7912d3f/lib/sprockets/bundle.rb#L39-L40