Add Work.RequestFunc (rakyll#149)

* Remove unused variable in test

* Add Work.RequestFunc

With this commit, people using hey as a library can generate different
requests, rather than repeating the same request each time. This can be
useful for simulating a variety of requests or replaying traffic.

Clarify -c and -q flag description (rakyll#190)

Switch to Go modules

Add request offset to csv (rakyll#126)