have_iptables_rule matcher support

Support checking iptables rules like this.

```ruby
describe 'iptables' do
  it { should have_iptables_rule('-P INPUT ACCEPT') }
end
```

You can also give table name and chain name like this.

```ruby
describe 'iptables' do
  it { should have_iptables_rule('-P INPUT ACCEPT').with_table('mangle').with_chain('INPUT') }
end
```