feat(require): add require option

Takes an object where the key is the custom dependency name and the
value is the file to require.
e.g.
    requires: {
      'angular': './vendor/angular/angular.js'
    }

[More information about
browserify#require](https://github.com/substack/node-browserify#brequirefile-opts)
Merge pull request #27 from guzart/add-require-option

feat(require): add require option