Memphis support and Pulsar improvements

Features

Plumber now supports Memphis!
Run plumber read memphis -h and plumber write memphis -h for flags

Improvements

New flags have been added for Pulsar courtesy of @ckreiling

• --token to specify authentication token as a string or file
• --initial-position to specify where the subscription should start reading from
• --listener-name to specify listener name when connecting to a Pulsar cluster advertising more than 1 DSN