Add a new feature to support run with multiple url

[homura]

This is a non-breaking-change feature

Why we need this feature

When we bench a distributed system across continents (such as a blockchain system), there is often no load balancing. At this time, if it supports natively multi-node bench, it will be very convenient.

How to use it

Added a compatible new feature to support multiple URLs like this

autocannon http://localhost:1234 http://localhost:1235

also programmatically supported with

const autocannon = require('autocannon')

autocannon({
    url: ['http://foo.bar:3000', 'http://baz.qux:3001']
})

[mcollina]

lgtm