Option to pass a custom hash string

If you specify the `customHash` option, it will be used instead of the MD5 digest for appending to filenames. So passing “my-hash” produces `assets/vendor-my-hash.js` instead of `assets/vendor-4e4d543cde3c8a73d4723711eca0796c.js`.

My use case for this is that elsewhere in my app I have a python web server serving up the entry point (`index.html`) for an ember-cli app, and there's some other stuff in there that prevents me from using the ember-cli generated `index.html`. I need an easy way in that template to know the names of the fingerprinted files. In the past I've used git commit SHAs as fingerprints (they are helpful in debugging production deploys too) which is what I would do with a custom hash option.

Let me know what you think.