add IPv4-mapped support to conversions

Hi, this should add IPv4-mapped support next to IPv4-compat.

```python
In [1]: from ipcalc import *
In [2]: ip = IP('192.0.2.42')
In [3]: print ip.to_ipv6('mapped')
0000:0000:0000:0000:0000:ffff:c000:022a
In [4]: print ip.to_ipv6('mapped').to_ipv4()
192.0.2.42
```

Thanks for the lib!