[WIP] Add connect-to flag

[AntonioMA]

Background

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

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.

[jfreeland]

this would be pretty handy

[upgle]

There is quite a difference in performance due to DNS cache. I need this feature too!

[tsenart]

Thanks for the contribution. I think I'd rather add the capability of Vegeta caching DNS responses for a configurable duration instead of this. It's my understand that would also solve your problem of overloading your corporate DNS server.