*** Allow filtering sources in current session

It is bound to C-M-e in helm-map.

*** Change `helm-current-directory-alist` default value

Now C-x C-f from a Gnus buffer brings to News directory.

*** Exclude commands related to specific modes in helm-M-x

This is available only in Emacs with an interactive version supporting

MODES argument. It is configurable with

`helm-M-x-exclude-unusable-commands-in-mode`, default to t.