Add a HopChain

[irl]

This chain will record details of ICMP TTL exceeded messages in the flow record including the full quotations for each ICMP message received. Unit tests and docstrings should be completed in the same style as for #148 and existing chains.

This will be useful for #103 and #111.

[britram]

@waltermi has done this in his own fork.

[irl]

#180