Create a `registry` command to manage the Event registry

[mattbishop]

This command manages the event registry for a ledger. With it, one can add, list and remove event types.

It drives Evently's /registry API entry point.

[evert]

Looking into the API a bit, this is what I'm thinking for the CLI, let me know what you think:

Creating a new event (long and short forms)

evently registry:new --entity foo --event bar
evently registry:new -e foo -n bar 

(I picked -n because they both start with an e and -n was the closest to name. We can also leave out the short option, or come up with something different. Most important is that we're consistent)

[mattbishop]

I would switch it. -e for event, -n for entity name.