Add scope chaining to fusebox.macros/scope

- If the first form passed to the fusebox.macros/scope macro is :chain
  the global fuse state will be used to initialize the state inside the
  macro block. Changes to the external state will not take effect inside
  the macro block.
Merge pull request #1 from chaptastic/scope-chaining

Add scope chaining to fusebox.macros/scope