Add support for MPTCP

[RuiCunhaM]

Add support for MPTCP as described in RFC 8684.

I've already implemented the dissection of the options for a personal use case, and I would like to continue the work and contribute with full support to Scapy. However since MPTCP is a more complex TCP option I was not sure what approach I should use to implement it fitting the already existing TCP options.

Therefore before carrying on I would like to know if the approach I used is valid or if a should refactor it, and if so, what is the best/correct way for doing it.

Thanks.