Allow cascading HTTP flow control

[amark]

Right now, the flow property for mapping routes allows apps to declare their sorting dynamically.

However, this is less useful as of now, because Coalesce will cascade to the lowest flow and then immediately writes a response. There should be a property that tells Coalesce on reply to continue to cascade, aka trickle down to the next map in the flow.

This would allow middleware to sit as their own process, rather than being required into the module with the lowest flow number. Plus it just makes more sense for it to work that way.

[amark]

I think this works now, but I really ought to write some tests (https://github.com/amark/coalesce/issues/4) to verify this. Won't close till then.