long string in flag.Usage
reset usage function
wip
using queue and job to download concurrently
per job channel to notify a job completed
separate job
renamed file
better ouptuing and encapsulation
use a map instead of an array in queue system
perint usage on stderr to get non zero exit code
add -v option
Merge pull request #1 from robinmonjo/jobs-and-concurrency

Jobs and concurrency