Add :array_append parameter to shellvar type/provider

See http://serverfault.com/a/557032/29328 for original idea.

### appending to arrays

    shellvar { "GRUB_CMDLINE_LINUX":
      ensure       => present,
      target       => "/etc/default/grub",
      value        => "cgroup_enable=memory",
      array_append => true,
    }

will change `GRUB_CMDLINE_LINUX="quiet splash"` to `GRUB_CMDLINE_LINUX="quiet splash cgroup_enable=memory"`.

    shellvar { "GRUB_CMDLINE_LINUX":
      ensure       => present,
      target       => "/etc/default/grub",
      value        => ["quiet", "cgroup_enable=memory"],
      array_append => true,
    }

will also change `GRUB_CMDLINE_LINUX="quiet splash"` to `GRUB_CMDLINE_LINUX="quiet splash cgroup_enable=memory"`.