Set prefix key before loading helm-projectile

Because helm-projectile loads helm-config, prefix key must also be set
before helm-projectile. Comment is added for future reference
Set Helm prefix key independent of Helm's packages

Since `helm-command-prefix-key' must be loaded before `helm-config', we
don't know if other Helm packages include `helm-config' and invalidate
custom prefix key. This change makes prefix binding independent from any
Helm package.

Conflicts:
	modules/prelude-helm.el
Merge pull request #684 from tuhdo/master

Set Helm prefix key independent of Helm's packages