Allow configuration via block

[mcordell]

Implies a slightly more flexible/robust configuration system.

See conversation here

[shanebonham]

Is this ready, or are you still working on it?

[mcordell]

Still open, but "solved" in that I have a solution written out in the configuration branch. But I am still testing:

1. I need to merge the next-rspec branch which sets out a bunch of tests for the existing implementation in next
2. Reconcile any broken tests with the configuration branch and write new tests for the new methods
3. Test it in a real app such as checkout

Then I would consider it ready