Add mangle table support

Add support for the "mangle" table type for iptables.

I tested this for my Jetty server, and it works for me.

This is quite similar to:
https://github.com/rtkmhart/cookbook-simple-iptables/commit/b09d31c46ebabcdf424356f9d4e877150e5745a1

It is different in these ways:
* I updated the README in a different way
* I did not include the fix for the [weird bug as mentioned in the other pull request][1], since (1) it seemed orthogonal to this pull request and (2) I did not experience it myself (that I know of).

[1]: https://github.com/dcrosta/cookbook-simple-iptables/pull/18