eventsAfter decryption

[taoeffect]

Problem

Currently there's no way for chel eventsAfter to decrypt messages.

This makes debugging applications difficult.

Solution

1. Create a flag called --keys <json-file> that takes a json file containing secret keys to be loaded that can be used to decrypt a chain.
2. Update the documentation to show how this file can be generated from an app that uses libcheloniajs.