introduce `--startup-time`, simplify `--run-time`

[jeremyandrews]

• document --startup-time
• calculate hatch rate if --startup-time and --users are set
• --run-time always starts after the load test is fully started
• fixes https://github.com/tag1consulting/goose/issues/219
• fixes https://github.com/tag1consulting/goose/issues/346

• including Starting, Running, and Stopping in time reports, for example:

  ------------------------------------------------------------------------------
  Users: 10
  Target host: http://apache/
  Starting: 2021-08-18 17:16:35 - 2021-08-18 17:16:37 (duration: 00:00:02)
  Running:  2021-08-18 17:16:37 - 2021-08-18 17:16:42 (duration: 00:00:05)
  Stopping: 2021-08-18 17:16:42 - 2021-08-18 17:16:42 (duration: 00:00:00)
  
  goose v0.13.2-dev
  ------------------------------------------------------------------------------