remove unused modules if they have sideEffects: false

[goto-bus-stop]

Following from #17, although we can't remove unused modules in the general case, if something does

var _ = require('lodash')
exports.a = function () { /* something not using lodash */ }
exports.b = function (x) { _.map(x, /* etc */) }

and you're not using exports.b, exports.b will be dropped, lodash becomes unused, and we can check the lodash package.json file to see that it is safe to remove the require() call.

So this is intended to address the case where all uses of a module have been shaken out, not for the case where it's not used in the first place—then it might be a polyfill or something.

[formula1]

Based off #17

I'm sure it would just add another thing on the todo list, but would it be possible to see if a file affects the window or global variable? Perhaps theres a way to ensure that the module is only exporting and not a polyfill. Big fan of browserify and how simple it is to use, want to see it at its best.