Pass opts.extensions to browser-resolve and resolve calls

This is done to allow requiring non-js files w/o extensions while using browserify, like `require('module')` instead of `require('module.coffee')` for CoffeeScript files. See substack/node-browserify#326 issue. Depends on shtylman/node-browser-resolve#12 pull request.

No tests for now, but I'll add some integration tests if you are willing to accept such functionality.