Add CashAddr specification

Summary:
Based on the Bitcoin Cash CashAddr spec at https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/cashaddr.md
Updated for eCash

Test Plan:
Read through the document, check that it's correct for eCash
Regenerated example addresses for eCash using the Python TestShell

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13824