Add connect-to flag

[dankegel]

Background

implement the feature requested in https://github.com/tsenart/vegeta/issues/575 and https://github.com/tsenart/vegeta/issues/691

This is just https://github.com/tsenart/vegeta/pull/576 rebased, with a slightly better usage message

Checklist

• [x] Git commit messages conform to community standards.
• [x] Each Git commit represents meaningful milestones or atomic units of work.
• [x] Changed or added code is covered by appropriate tests.

[tsenart]

https://github.com/tsenart/vegeta/releases/tag/v12.12.0