adds optional js manifest for extending functionality via third party gems

Hi there. I wanted to extend rails_admin to add a new type of field. Specifically- though not really important to this pull request, I wanted to extend the `Text` type with markitup-rails. When I opened up rails_admin to take a look for the class I wanted to extend, and how I might go about that, I noticed there was a mess of custom extensions/plugins in the field types. To me this does not seem healthy for the project (it bloats the gem, it adds a lot of maintenance requirements for you, and it just doesn't seem like it will scale well over time), and I am not sure if I am missing something or if this was just how things were going at first.

So, I started poking around to see if it would be possible to extend rails_admin via a third-party gem of my own design. It was. While doing that, the only issue I really came up against was good way for getting javascript encapsulated in my gem into the rails_admin js manifest at run-time and in the right order. So, I added a new manifest, required it after everything, and then you can just overwrite it in your project if you want to add custom plugins that extend rails_admin.

If there is a better way to do this that I just missed, I would love to hear it.

You can see the current source code for the gem at https://github.com/Universum/rails_admin_markitup, the readme also explains how it would be implemented. It's dependent on my forked version of rails_admin right now, so I haven't released it to be bundled.

Anyway, would love to hear some thoughts/feedback.