debug: add custom "formatters"

So that the browser, node, and possibly the end users can
define their own "%n" formatters for the "format" argument
while using debug.
browser: map "%j" to `JSON.stringify()`
node: map "%o" to `util.inspect()`
debug: remove hacky placeholder logic

@visionmedia pointed out that it is not necessary, which is
good because it was very hacky.
Merge pull request #100 from visionmedia/update/add-custom-formatters

Add custom "%n" formatters