Enable prelude-global-helm-global-mode by default 

Only activates uses it when prelude-global-helm-global-mode is activated.

**Updated**:  Since Helm is another option to Ido, users who want to use it fully
should benefit from the default Prelude setup. If some users don't like, they can always disable global Helm mode according to the instructions in the homepage.

@tuhdo `helm-descbinds` should also from the prelude required package list.