Add support for --src-type and --dst-type

Add support for filtering by source and destination address types. Supported by
iptables only, this feature allows filtering packets by the address type (such
as whether the packet came from a local address, a broadcast address, a
multicast address, etc).

Adds the `:address_type` feature to allow a provider to declare support for
filtering on the basis of address type.
Rename {source,destination}_type to {src,dst}_type

As suggested by @dcarley.
Merge pull request #212 from alphagov/addrtype

Add support for --src-type and --dst-type