Add support for disabling autoprefixer

[kimroen]

Fixes #6

You can now disable autoprefixer by passing in enabled: false to its options. Autoprefixer is disabled by default.

For instance to disable in development, you could do something like this:

// Brocfile.js
var app = new EmberApp({
  autoprefixer: {
    enabled: process.env.EMBER_ENV == 'production' ? true : false
  }
});