Add support for :scripts: (pre/post) for subpackages

With this commit we can add pre/post/(etc) scripts in gem2rpm.yml for
subpackages, as in the example below:

:custom_pkgs:
  apache:
    :scripts:
      :pre: |-
        some_command
      :post:
        another_command
typo in gem2rpm.yml.documentation: :custom_pkgs: instead of :custom:
Merge pull request #1 from tampakrap/scripts_subpkgs

Add support for :scripts: for subpackages