Allow Browserify or Babel to be disabled from the babelify function

[frastlin]

Hello, I love this middleware, but it is a little difficult to debug my code when both browserify and babel are running. (I keep getting: "SyntaxError: expected expression, got '<'" which means that in my main.js file, in the module test.js there is a missing brace). At the moment I really only need browserify to develop with, but eventually I will need both, so I would like to be able to disable one or the other. Thanks,