Make plugins AMD modules.

[dev-tim]

It would be good to make d3 plugins defined as AMD modules to make it possible to use it with requirjs

[danse]

I think that the idea of plugins is exactly to avoid this complexity. The problem is that the javascript community is divided about how to implement modules

[vicapow]

I would personally prefer them to be commonJS

[vicapow]

Or at the very least, allow "d3" to be an optional dependency instead of always requiring the "d3" object be global.