Good start!

Some useful feature additions would be:

(1) Tunable number of requests per connection: it's often useful to be able to see how webserver performance differs when multiple requests are made per connection (Connection: Keep-Alive) vs. a single request per connection (Connection: Close).

(2) Tunable concurrency: webservers have to serve requests from different clients concurrently. It's much more illuminating to see how a webserver performs when multiple requests are coming in simultaneously than sequentially. Simply divide the total request rate by the number of threads/goroutines created, and assign each the fractional request rate.

(3) The ability to specify custom request headers. Enough said.