Include a load tester in the Joystick CLI for joystick test

This would work like joystick test load --requests 123 --duration 30 (where duration is in seconds). Requests would just be spread out evenly over the duration.

I'd want to be able to see:

• Each request that's run (verbose mode?)
• The timeline (animated so I can get a feel for race conditions and o(n) problems)
• Any payload that's past via the body

Ideally, we do this as scripts that can be run via joystick test load --script scriptNameInTestsFolder. That way you can write individual load tests and get the results back in a single report.