allow dynamically configuring users during running load test

[jeremyandrews]

• accept the users controller command during a running load test
• allocate new users on-the-fly if the number of users is increased during a running load test
• test dynamically increasing and decreasing users during a running load test
• users are increased/decreased per configured hatch_rate (or at a default of 1 per second)
• add book page with example graph (use mdbook build src/docs/goose-book --open to preview)
• builds on https://github.com/tag1consulting/goose/pull/464 which must be merged first
• closes https://github.com/tag1consulting/goose/issues/454