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.
This is `setq` `append` is quite ugly, and can be better accomplished with `nconc`. And I suggest you to put the comment on its own line. Use one semicolon for inline comment consistently.