Implement `get_network_address/2`

Write a function that take an ip address and subnet mask as input and returns the network address

```
iex> get_network_address("192.168.0.18","255.255.255.0")
"192.168.0.0"

iex> get_network_address("192.168.0.118","255.255.255.128")
"192.168.0.64"
```