Improve default behavior

[yoshuawuyts]

I ran into #7 again today, which made me wonder if we might improve the default behavior or envify.

Instead of having to choose between require('envify') and require('envify/custom') this behavior might be more intuitive.

var envify = require('envify');

// detects the process.env.NODE_ENV variable
browserify()
  .transform(envify())

// manually override the environment
browserify()
   .transform(envify({env: 'custom'});

Let me know what you think, if you approve I'll PR the changes.

[hughsk]

So the reason for this API being as it is right now is to cater to transforms specified from either the CLI or within a package.json file: in this case, you're expected to export a function with the transform signature, i.e.:

module.exports = function(file, opts) {
  return new Stream
}

So in the above example you'd actually have the following, where envify is passed in directly:

var envify = require('envify')

browserify().transform(envify)

The only way to get the behaviour you're after is to check the first argument for an Object vs. String, which could be a bit quirky but might work! Would definitely want this to be a major version bump though. If you wanna give it a shot send a PR and we'll see :)