Improve Helm configuration

The current prelude-helm only uses a single command of Helm, which
is a waste given how Helm contains many other commands. When
prelude-helm is activated, users should be able to utilize all of these
useful commands. If a user wants to use Helm, he will want to use all of
its features anyway.

`prelude-helm` is configured that it is able to be used with this [guide](http://tuhdo.github.io/helm-intro.html). If this got merged, I can put a link from my guide for readers to download and use Prelude for trying out Helm.
**C-c h** should be `<kbd>C- c h</kbd>`. Same goes for other keybinding references.