Allow to add external references to browserify stream

[josescgar]

Allow to specify which libraries should not be included in browserify builds so they can be provided externally, i.e.: vendor.js.

[jonjrodriguez]

I was just about to submit a pull request for this functionality using options instead of something on the browserify config object. With this method, how would you build the vendor.js?

With it as an option, it works like below.

mix.browserify('main.js', null, null, {require: ['jquery'])
   .browserify('second.js', null, null, {external: ['jquery']);

With it on the config object, how would you build vendor.js or main.js? The below wouldn't work

elixir.config.js.browserify.externals = ['jquery'];

mix.browserify('main.js', null, null, {require: ['jquery']); // jquery not included here
mix.browserify('second.js');