Split code into bundles with browserify.external

With this pull request you can split your app in more that one bundle and mix and match them as you find suitable. For example you can split your code into two bundles and load them in parallel to speed up js loading. Or you can split your code into "shared code", "code specific to page 1", "code specific to page 2", etc. bundles and then load it only required parts on each page.

Sample mimosa-config
libs = ["/vendor/js/jquery/jquery", "/vendor/js/bootstrap/bootstrap", ....]
browserify:
    bundles:
      [{
          entries: libs
          outputFile: 'lib-bundle.js'
      },{
          entries: ['js/app.js']
          outputFile: 'app-bundle.js'
          external: libs
      }]