Attack option: number of workers

In the context of a perpetual attack, where the duration of the attack
is set to a very long time, scheduling massive amounts of go-routines
isn't very efficient. This change set enables the specification of the
concurrency level of the attack by number of workers.
The default `0` matches the old behaviour of using one worker per hit
(request).
Handle signals in attack command
Update documentation of defaults
Implement collect command

This command concurrently reads multiple result streams and serializes
them into one output stream.
Merge collect and report cmds
Add Stop method to Attacker
Stop attack after signal
Pass workers option though Attacker