alternative configuration method

[dominictarr]

another response to #132

require only takes a single argument,
so what if supplying a second argument was a directive to browserify?

this would give us the desired configurability, and the advantage over #174
is that the configuration would fall in front of you eyes, not off in a file where you
would forget about it.

//load './client' when bundling for client side.
var server       = require('./server', './client')   

//ignore './server-only' when bundling for client side
var onlyServer = require('./server-only', null) 

I'm not sure how everyone feels about adding a second argument, but I think, all things considered,
this might be the cleanest, easiest to use method.

@isaacs has stated that the module system is "finished", and I think he's pretty serious,
so it's unlikely that it would suddenly start behaving differently with a second argument.

[ghost]

This functionality is now handled by the browser field.