Allow enable/disable global helm key bindings

Since some users prefer using default Prelude commands, a new global
mode is defined: prelude-global-helm-mode. When activate, Helm binds
some global key bindings to its own commands. When deactivate, Helm
removes the bindings and Prelude uses the default bindings.
Merge pull request #657 from tuhdo/master

Allow enable/disable global helm key bindings