Add option 'helm-apt-cache-show-function'.

Add option `helm-apt-cache-show-function' to make user can customize "show description" interface.

I use below code instead `helm-apt-cache-show':

(setq helm-apt-cache-show-function 'apt-utils-show-package-1)
Merge pull request #390 from manateelazycat/patch-2

Add option 'helm-apt-cache-show-function'.