Check if target host is reachable before the load test

[kffl]

As of now, load test target validation includes some checks (URL parsing being successful, protocol being supported and the port being present), but there no additional steps undertaken in order to determine if the target host is actually reachable (i.e. does its domain name DNS query resolve properly?; can a TCP connection be established?). Consequently, if Gocannon is unable to connect to the host, the user will find out about that fact only after the load test ends (with none of the requests being completed successfully).

This feature could be implemented by TCP dialing the target host prior to the load test start and returning a descriptive error if such operation is unsuccessful.