Custom theme via prelude-theme variable

The problem with the current approach is that even if we call `disable-theme` explicitly, there still will be some leftovers with the previous loaded theme. In the case of zenburn for example, `smartrep-mode-line-active-bg` get set to "#383838", when you disable zenburn, it's still "#383838", when you load a light theme, it looks like hell.

![screen shot 2014-10-11 at 12 04 42 pm](https://cloud.githubusercontent.com/assets/2468952/4601395/bd9c7e04-50fb-11e4-92f6-497d526a59a5.png)

Now, the user can load their preferred theme via setting the variable in their preload personal configurations like:

```el
(setq prelude-theme 'sanityinc-tomorrow-day)
```