Add support for ES6 import syntax

This might be a long shot but I'd like to see this in browserify.

The ES6 module syntax has a few nice features compared to commonJS `require` method,
namely [bindings](https://github.com/esperantojs/esperanto/wiki/Bindings) and [cyclic dependencies](https://github.com/esperantojs/esperanto/wiki/Cycles).

Since these things are in the ES6 standard it seems reasonable to work them into browserify too!

Note that just using a ES6 to ES5 transpiler wouldn't work in this case because these are cross module concerns.
For instance, `babelify` is only able to transpile `import foo from 'foo'` to `var foo = require('foo')` at file-level, but this does not allow cyclic dependencies.

The [Esperanto](https://github.com/esperantojs/esperanto) project has a nice way of handling things but lacks too many features to be a valid replacement for browserify.

Is there any possibility to see support for ES6 `import` syntax come to browserify?